Original subtitles

[dramatic music]

- [Narrator] Fear stalk the streets of San Francisco.

[gun bangs]

- This guy is a pathological, psycho killer.

- [Narrator] As the death toll mounts,

the killer sends out coded messages.

- It's like something from a horror movie.

- [Narrator] He calls himself the Zodiac,

but could cracking the killer code reveal his name?

- He sets up this one by saying, "My name is."

- [Narrator] But every new code is more complex

than the last.

- The total number of permutations would be bigger

than the number of atoms in the universe.

- [Narrator] Now, 50 years on, there's been a breakthrough.

- Kind of chilling.

Here were the words of the Zodiac being heard again

for the first time in half a century.

- [Narrator] These are the codes that changed our world.

Bizarre markings, random letters and numbers,

words that make no sense,

but cracking them unlocks military secrets,

decodes ancient civilizations,

[gun banging]

and reveals enemies in our midst.

[dramatic music intensifies]

Now we uncover how they were decoded,

the genius minds that broke them,

and the secrets they reveal. [logo scraping]

[tense music]

San Francisco, Northern California, 1968,

the week before Christmas,

the bodies of teenagers, David Faraday and Betty Lou Jensen

are discovered on a secluded lane

in the North Bay area of the city.

[gun bangs]

David has been shot in the head.

[gun bangs]

Betty Lou has been shot in the back five times.

- The boy was shot right at the side of the car,

and the girl apparently tried to run.

[Betty Lou yells]

And she was shot and found 28 feet further on.

[gun bangs]

- From the very outset,

it was clear to the police that they had a serious problem.

The first couple discovered shot multiple times lying next

to their car in the most innocuous of all places,

a make out spot.

[tense music continues]

- One of the first assumptions is robbery.

You know, just somebody pulling up

and robbing these two people and somehow it went south

and killed them both, but nothing was taken.

- The whole community was in uproar. It was out of the blue.

No one knew what happened or why.

- - [Narrator] Despite media coverage,

detailed forensic work and intensive interviews

with a couple's family, friends and acquaintances,

the police fail to make any arrests.

- One of the first questions the police ask

whenever they come across a murder

is what is the motive?

The vast majority of murders are committed by people

who knew the victim, a friend, a business associate,

a family member.

So they're gonna look for some connection to the killer,

but there seems to be no apparent motive for the killings.

- [Narrator] With no leads, the trail quickly goes cold,

[eerie music]

then less than seven months later, there's another shooting,

just a few miles from the first crime scene.

The victims are 22-year-old Darlene Ferrin

and 19-year-old Mike Mageau.

The MO is disturbingly familiar.

- You have a couple parked in a secluded location,

approached and brutally assaulted, again,

with no apparent motive.

- The car pulls up behind them.

A lone guy gets out, opens fire.

[guns banging]

He got back in his car and then drove away.

- [Narrator] 40 Minutes after the shootings,

the nearby Vallejo Police Department

receives an anonymous call.

- The caller has detailed information about the crime.

He's able to describe the location, the color of the car,

and even the weapon used in the murder.

A nine-millimeter handgun.

- [Narrator] These are details only the killer could know.

- Police are now thinking, "How do you know this?

You must have been there. It can't just be a prank call.

This can't be a coincidence."

- [Narrator] And the caller

makes a second chilling confession.

- He claims responsibility

for the killings seven months earlier,

but before the police can trace the call, he hangs up.

It's clear to them, though, that the same man is responsible

for these crimes.

[dramatic music]

- [Narrator] As news of a second double shooting spreads

across the North Bay area,

local journalist Duffy Jennings senses the panic spreading.

- The fear and the tension started to build

because of the media exposure.

- [Narrator] The one difference

between the killers first and second attacks

is that this time there's a survivor,

[gentle dramatic music]

but 19-year-old Mike Mageau is so traumatized,

the police still have little to go on.

- The description is so vague.

It really doesn't leave the police with any clues

that they can follow successfully.

- He's had such a traumatic experience that the things

that he'll remember from that moment will blur into one.

He was shot. He was left for dead.

[dramatic music]

- [Narrator] Two brutal attacks, seven months apart.

And there's still little progress in finding the killer.

- We don't know who he is. We don't know why he is doing it.

We don't know where he is gonna shoot somebody next.

It was a real mystery.

- [Narrator] The trail for the serial killer goes cold.

[upbeat music]

Then eight months after the killing spree began,

there's an astonishing twist.

Someone claiming to be the killer sends handwritten letters

to three local newspapers.

- This is a really unprecedented set of circumstances.

A killer that is openly communicating

about the crimes they've committed.

- [Narrator] The letters contain unpublished details

that only the police and the murderer could know.

[suspenseful music]

And each letter carries the same strange sign off.

- When the killer wrote to the newspaper,

signing off with this symbol,

which was a circle with cross lines in it,

everyone immediately took that to mean the crosshairs

of a rifle scope.

- [Narrator] The killer sends more than just the letters.

Each of the newspapers also receives a sheet of paper

covered in strange combinations of letters and symbols.

- There were circles, circles with lines through them,

triangles and everything

had this crude, handwritten quality.

[dramatic music]

- [Narrator] It is the killer's personal

and very deadly code,

containing more than 400 letters and symbols.

- Police are not trained in espionage.

This is not their area of expertise.

So when they receive this code,

it's just outta their league.

- [Narrator] To focus police attention

on his mysterious code, the killer promises

that cracking it will reveal his identity.

- Police work is based on the specific methodology,

forensics and looking at the crime scene

and speaking with witnesses.

And now they're getting

this mysterious code directly from the killer themselves.

They just don't know what to do with it.

[siren blares]

- [Narrator] The police reach out

to the FBI's top cryptologists,

but cracking the strange code

isn't the investigators' only problem

because the killer's letter also contains a threat.

"Print a section of the code on the front covers

of the three national newspapers

or the body count will keep rising."

- The newspapers are the thing that everybody reads,

that everybody will see.

You're gonna get it out to a lot of people

to make sure that you are noticed.

He's letting people know that he's here.

- [Narrator] In a bid to prevent any more deaths,

the decision is made to concede to the killer's demands.

The coded messages are published,

but the police publicly cast doubt

on the messages' authenticity

and request more from the letter writer

to prove he's the killer.

- On the surface they're saying,

"Prove that you're telling the truth,

prove you are who you say you are,"

but really they're just trying

to get the information necessary to catch the guy.

- They need him to fall over a bit

so that they can catch up with him.

[dramatic music]

- [Narrator] The plan to goad the killer

into responding, pays off.

He sends another letter.

This time, his opening line is haunting.

- [Zodiac] Dear Editor. This is the Zodiac speaking.

- For the first time,

he identifies himself as the Zodiac Killer.

And that's how he'll be known from here on out.

- The Zodiac Killer is now forcing his own narrative.

He's taking control of his new persona

and his new personality.

He is the Zodiac, he's here and he's building his brand.

- And by giving himself a title, he's handed the press,

journalistic gold, they have a villain

that they can write stories about,

and that name will soon become a household name.

And you have to think that with all his actions,

this is exactly what the Zodiac Killer hoped for.

- First of all, he's saying,

"I'm smarter than anybody else."

And second, it became kind of a game to him while people

are lying dead and their families

are mourning their dead relatives.

This guy's getting a kick out of whatever he's sending.

[eerie music]

- [Narrator] Once again, the letter contains details

only the killer could know, and he taunts the authorities

for their failure to crack his code.

- He doesn't really think they're going to break the code.

And he's just gonna sort of tease them a bit about it.

Again, trying to assert that he's the superior person here,

that he's really running the show, not the police.

- He's really enjoying this cat and mouse game.

He's sending out something, the mystery,

the legacy that he's leaving.

He's really enjoying making people confused

and making people think and making people fearful.

That is exciting to him.

- [Narrator] The Zodiac's latest letter

doesn't bring the police any closer

to working out his identity,

[mysterious music]

but making the code public has an unintended benefit.

The police and FBI cryptographers are joined

by hundreds of amateur code breakers across America,

all searching for ways to break the Zodiac's cryptic code.

- Zodiac Killer uses a cipher that seems

to be replacing each letter with some kind of secret symbol.

And these are symbols that apparently he's invented.

The first step in decoding it

is just to figure out what type of cipher it is.

- [Narrator] The patterns of symbols lead many code breakers

to suspect the Zodiac is using a variant

of a code dating back to the first century BC,

[tense music]

a substitution cipher.

[lively dramatic music]

Substitution ciphers were used

by one of the most famous Roman leaders of all time.

- Turns out that Julius Caesar

was quite a keen cryptographer.

Caesar would shift every letter up three places.

So a becomes D, B becomes E, C becomes F, and so on.

And that is the classic Caesar cipher,

shifting every letter up three places.

But nowadays, when we talk about a Caesar cipher

we're we're just saying shift up letters,

any number of places.

[dramatic music]

- [Narrator] The key to cracking

this basic substitution cipher

is knowing this shift pattern.

In its most basic form, the Caesar cipher is limited

to the number of other letters in the alphabet

other than itself, giving 25 different possibilities.

- The problem with a classic simple substitution cipher

is that you can break it with frequency analysis.

So by that, I mean, some letters

are more common than others.

- [Narrator] In English, the letter A accounts

for 8% of all letters in a typical page of text.

E accounts for 11%. T appears 7% of the time.

And Q just 0.1%.

That means if all letters are substituted

by just a single symbol,

working out the percentage they appear within a text

will reveal which letter each symbol represents.

- And soon or later, probably sooner,

we'll find out what the message says.

[eerie music]

- [Narrator] The FBI cryptologists

and the amateur code breakers suspect

the Zodiac is using a substitution cipher.

But despite that, this code is stubbornly difficult to crack

because the Zodiac has added a complex twist

to his coding method.

- It's immediately clear that this

is not a one-to-one substitution of the English alphabet

because in the English alphabet, there are only 26 letters.

- Even if you add in the numbers from zero to nine,

the numerals, it's still not enough.

There's something else going on.

- [Narrator] There are more than twice the number of symbols

in the Zodiac's code than there are letters in the alphabet.

Cryptologists suspect he's using more

than one symbol to represent the same letter.

- For example, for the A you might have the number one

or B the letter M or another symbols.

So you have several symbols to choose from.

- [Narrator] And that means frequency analysis on its own

isn't enough to break the Zodiac's code.

- Because there's nothing that's standing out

because there's no obvious variations of frequencies,

it's very hard to then begin to decipher that message.

[upbeat music]

- [Narrator] As efforts to crack the Zodiac's code continue,

the unknown killer remains at large,

the feeling of terror across San Francisco intensifies.

- So releasing this code to the public,

it's a bit of a double-edged sword.

On one hand, you're enlisting this vast knowledge base

to help you solve this mystery.

But at the other hand, you're giving something

that's already received press even more press,

and you run the risk of greatly exacerbating the fear

that the general population already

is suffering from about this guy.

- [Narrator] Two weeks after the Zodiac's coded message

is published, parts of the city feel

like they're verging on hysteria.

The police are at a total loss.

[siren blares]

[gentle eerie music]

Then a breakthrough comes from left field,

two crossword puzzle enthusiasts,

get in touch with the police.

- Some of the greatest code breakers have been people

who also love crossword puzzles and word games

and number games because you have to have a mind

that's able to focus on the details,

but also think about kind of crazy intuitive,

out-of-the-box possibilities.

- [Narrator] Teacher Donald Harden and his wife Bettye

claimed to have done what the best minds

in the FBI couldn't,

they say they've cracked the killer's code.

- Bettye and Donald Harden became intrigued

by the Zodiac puzzle, like thousands of other people.

And almost as if they were sitting down

to solve a Sunday crossword puzzle, they went to work on it,

but they worked on it with a really smart plan.

They knew they had to find some set of characters

they could connect to a real word,

something that frequently repeats in the document.

This is a classic technique for code breaking.

You look for what's called a crib,

some combination that keeps popping up.

- [Narrator] The Hardens have used exactly

the same techniques created more than three decades earlier

by some of the greatest code breakers in history.

[dramatic music] [boots clicking]

During World War II, the German Army relied

on a secret code to pass messages onto Nazi forces,

generated by their now legendary Enigma machines.

The encrypted messages were thought to be impenetrable.

- The Enigma code is a substitution cipher.

You swap one letter for a different letter,

but the difference with the Enigma cipher is its dynamic.

The same letter is being encrypted

by a different letter differently,

each time you hit the key.

- [Narrator] The Enigma machine's dynamic encryptions

made British code breaking efforts almost impossible.

- There are millions upon millions of different permutations

that are going on here.

And unless you know the millions upon millions

of permutations, you've got no idea

what this code really means.

There's no way of going back to the original message.

- [Narrator] The Germans made cracking Enigma even harder

by changing its settings every 24 hours.

- It would've taken an army of analysts decades

to crack the code.

[dramatic melodic music]

- [Narrator] British code breaking efforts

were only saved by the discovery

of a critical design flaw in Enigma.

It couldn't code a letter as itself.

- If I type the letter A into an Enigma machine

a lamp board lights up with another letter,

that letter will never be A, if I type B,

B will never light up, C, C will never light up.

It narrows down all the possibilities.

- [Narrator] It was a critical breakthrough

for British code breakers.

They just needed to find a way to exploit their discovery.

- So they look for things called cribs,

which are essentially repeating words.

So this might be, Heil Hitler,

which was used frequently as a sign-off.

- If we can match one word

that you've guessed in the message

with a bit of the encrypted message,

then you can begin to pin down the settings of the machine.

- [Narrator] Enigma settings were changed at midnight.

So to give them as much time as possible,

the British code breakers hunted for cribs

to Enigma's daily codes

in one of the most mundane, early morning transmissions,

a weather report sent out at 6:00 am every day.

- So a weather report was particularly useful

because it would be a daily transmission

that they would make, it was fairly routine.

So any repeating pattern like that was vital.

- [Narrator] British code breakers

searched the early morning code for strings of characters

that could disguise the words for weather report in German.

- So let's imagine I've intercepted a message

from weather station.

And I think it's got the word wetter in it,

W-E-T-T-E-R, the German word for weather.

It must be in there somewhere.

It's a weather station report.

Well, is it at the beginning of the message? W-E-T-T-E-R?

Well, no, because the first letter of the encrypted text

is a W and I know no letter can be encrypted as itself.

So I shift the wetter along a little bit

until I find the word wetter encrypted

with six completely different letters.

Now that doesn't mean that's the right place for wetter,

but it tells me that it's potentially the right place.

- So if they identified a word that was repeating,

such as weather report,

that would give them all of those individual letters,

which was a massive key to then unlock

the rest of the day's coding.

It was a huge advantage.

- [Narrator] Weather report was the first of many cribs

that British code breakers correctly deduced,

all these cribs were key to cracking Enigma.

It provided British intelligence with some

of the German military's most closely guarded secrets.

- Once you know how these messages work,

it's very easy to guess the cribs and you are cracking them

more and more quickly as the war progresses.

- All of this information was absolutely vital

to the Allied war effort.

People now looking back say,

"Perhaps it shortened the war by even up to two years."

[bomb explodes] [suspenseful music]

- [Narrator] 30 Years later,

crossword enthusiasts Donald Harden and his wife Bettye

apply the same crib hunting technique to try to crack

the Zodiac Killer's code.

- So the Hardens thought, "Well, this guy's a killer.

He's bragging about his killing.

What are some of the words he might be using in a document?"

- [Narrator] The Hardens begin their search

by looking for patterns in the symbols

that could represent a very obvious word choice

for a serial murderer, kill.

[dramatic music]

And slowly the Zodiac code finally begins to crack.

The first step is spotting a character

that appears more than once, like the half filled squares.

- They spot some symbols that repeated.

So that could be the double L in kill.

- [Narrator] If these represent double L,

then the diagonal line could be a K and the triangle

could be an I, completing the word kill.

- The eye is really useful

because it's the first letter of the message.

And as part of the guesswork of figuring out

what this killer is trying to say,

it really stands to reason that I would be

the first word of the message.

- [Narrator] The symbol the Hardens believe

is the letter I match and a possible first line

begins to emerge.

- The first line of the Zodiac killer's message

was the key for the solution

of the how the two broke into the cipher.

- [Narrator] But the killer has used far more symbols

than there are letters in the alphabet.

So the Hardens know each letter

has more than one substitution,

just like solving a crossword,

they use a mixture of logic and educated guesswork,

until every word in the first line is revealed,

and eventually, the entire message.

[dramatic music continues]

A pair of crossword-loving amateurs has succeeded where some

of the country's most elite code breakers have failed.

The message confirms just how twisted the Zodiac is.

- The deciphering starts,

"I like killing people because it's so much fun.

It's more fun than killing wild game in the forest

because man is the most dangerous animal of all."

- When the decrypted message was released,

it had probably exactly the result the Zodiac Killer wanted.

It just horrified people,

someone who was taking this perverse pleasure

in not only killing people,

but then bragging about it to the public.

- [Narrator] Despite the killer's promise in his letter,

the message the Hardens reveal does not identify him.

The police are back to square one.

- When they do crack this first code,

all they really learn is that the killer

is taking great pleasure from his acts

and that he's not going to stop anytime soon.

[dramatic music]

- [Narrator] The Zodiac is toying with the authorities

and they know it.

The police are no closer to identifying him

than they were the first time he killed.

This does nothing to calm the fear rippling

across the Bay Area of San Francisco.

- There is a killer on the loose, he enjoys killing.

He strikes seemingly at random and without warning

and the police have no idea who he is.

- "Is he coming for me? Am I gonna be targeted?

What does he want?"

I don't understand all of this will be going through

the community's his mind.

- It's horrifying to think that this person

could be out there walking among us,

looking for the next opportunity

to play this really sick game.

[dramatic music continues] [siren blares]

- [Narrator] With the police no closer to catching him,

the killer is about to ramp up the fear

already infecting San Francisco.

[eerie music]

September 27th, 1969,

less than three months after the second Zodiac killings,

[phone rings]

the Napa County Sheriff's Office receives a phone call.

It is from the Zodiac Killer.

He has struck again,

and he wants to be sure the police know it was him.

- The caller at the time couldn't know that the police

had already been tipped off by a local fisherman.

So they were on the scene.

They were already investigating the crime,

when this call comes in.

- [Narrator] At Lake Berryessa, north of San Francisco,

college students Bryan Hartnell and Cecelia Shepard

have been stabbed multiple times and left for dead.

The latest attack carries many of the hallmarks

of the Zodiac, and this time,

the attacker has left behind a calling card.

[creepy music]

- There is the message left on the side of the victim's door

with the dates of the Solano County murders and ours,

along with other items that are on there,

have definitely indicated to us

that they're one and the same.

- This is a killer that is going

to spend time setting up a scene.

He's communicating a message

to whoever's going to discover his victims.

- [Reporter] Police and park rangers combed the scene

for clues until late yesterday,

it was here the couple was picnicking.

And here a man in a mask robbed,

tied and stabbed them, leaving them for dead.

- [Narrator] The Zodiac

has now attacked six innocent people,

but once again, the killer has slipped up.

When the police reached the scene at Lake Berryessa,

both victims are still alive.

- [Reporter] Sergeant Bill White was first on the scene.

- The guy had told him, he said, "Take the money."

He said, "I don't want it.

All I want is to kill you people, I have to kill you."

Well, he started stabbing the kid in the back.

Real nice college kids, just stabbed for no reason at all.

I never witnessed anything like it before.

- So Cecelia is able to give some clues

as to the appearance of the Zodiac Killer,

but her wounds prove fatal and she doesn't survive.

So the Zodiac Killer has attacked six people,

four of them have been killed,

and still the police, beyond this vague description,

have no idea who they're dealing with.

- [Narrator] Any hopes for a detailed description

of the Zodiac Killer, now lie with Bryan Hartnell.

He has been stabbed six times, but miraculously,

none of his injuries are life threatening.

For the first time,

police have a full description of the attacker.

- Can you describe for us what your attacker looked like?

- He was a medium to short height.

He had this black hood on, came clear down to here,

just little slits in the eyes

and wears these clip-on glasses.

- [Narrator] An artist's impression of the Zodiac

based on Bryan Hartnell's description

is released to the public.

[suspenseful music]

- Finally, terror has an image to go with it.

- [Narrator] But because the executioner-style hood

completely obscured the killer's face,

it doesn't move the police hunt.

[siren blaring]

On there is still a killer on the loose.

And no one is certain what he looks like

or where he will strike next.

- This guy is a pathological, psycho killer.

There's no doubt about it.

- Day to day, you weren't sure where the next

random violent attack was gonna come from.

[tense music]

- [Narrator] Just two weeks after the stabbings

at the lakeside, the Zodiac Killer ups the stakes again,

All his previous victims have been found in remote spots

on the city's outskirts,

but the killer is clearly growing in confidence

or at least in his thirst for publicity.

[gentle dramatic music]

When taxi driver Paul Stine is discovered shot dead

in his car in Presidio Heights,

a busy neighborhood of San Francisco,

it is such a different MO the police

don't immediately connect it to the Zodiac Killer.

- So when they got there and found the driver

with a bullet in his head, blood all over the place,

that assumption was that it was a robbery.

And so they just conducted it

as a regular homicide investigation.

[lively dramatic music]

- [Narrator] That all changes when the newsroom

of the "San Francisco Chronicle" receives a package

containing a piece of Paul Stine's blood-soaked shirt.

Packed in with the shirt is a third letter

with the now disturbingly familiar symbol at the end.

- It was the first and only known time that he struck

in the city of San Francisco.

And it broke his MO of attacking young couples

in lover's lane or picnic situations

as he had done three times prior to that.

So now all bets are off.

- [Narrator] The Zodiac's latest letter suggests there

are now no lengths he will not go to,

to increase his notoriety.

The letter says, he's come up

with a whole new kind of killing spree.

This time, it will be children leaving their school bus.

[eerie music]

- With the letters threatening more killings,

threatening to shoot kids coming out of a school bus,

now the entire region is gripped in this fear of

the unknown potential killer. [siren blares]

- Police respond extraordinarily.

They put out extra patrol cars.

They assign police to monitor the school buses.

- [Reporter] The bus patrols started first

in this county Wednesday.

Police units from various agencies using marked

and unmarked cars, follow buses on their runs

to and from schools.

- They're trying to do everything possible

to prevent the attack that the Zodiac Killer has threatened.

- [Reporter] The massive patrols are responses

to the Zodiac Killer's threat to stop a bus

and shoot children.

- [Narrator] Local beat cops work up a picture

of a man seen near the Stine killing.

Could this be the face of the Zodiac Killer?

If it is, it still produces no leads.

For 10 months, the Zodiac Killer

has terrorized the San Francisco area.

He has brutally slain five people,

and seriously wounded two more.

Although the killings and threats have continued,

one thing has stopped

since his first elaborate Caesar cipher,

cracked by two crossword buffs,

there have been no further codes,

[dramatic music]

but three weeks after the murder of taxi driver Paul Stine,

a new code is delivered to the "San Francisco Chronicle."

And this one is far more complex

than the Zodiac's first attempt.

- The killer realized that his first message

was broken pretty soon.

So he added another step of complexity to the second one.

He probably still wanted it to be solved,

but he wanted to make it harder.

- [Narrator] The Zodiac's new cipher

contains 63 different symbols.

Nine more than is first.

And the message is 80 characters shorter.

These two changes make cracking this code,

a monumental challenge.

- The shorter the message, the fewer words

are gonna be repeated.

The fewer opportunities you have to check

what might be a common substitution.

- So it's clear at this point that whoever

is conducting these killings is an intelligent individual,

that he has some type of plan in mind.

And now, he's raised the complexity of things

with his new code.

- You get the sense that he was taking

this perverse pleasure in this game.

- [Narrator] Amateur and professional cryptologists work

around the clock to try to crack

the Zodiac's latest message.

Over the next five months,

the killer sends a further two letters to the newspapers,

mocking the police further for their failure to catch him

or crack his latest code.

- It's almost as if he got more of a kick out of hiding

in plain sight, if you will,

almost than the actual murders themselves.

[soft dramatic music]

- [Narrator] 14 Months

after the Zodiac's killing spree began,

the police still have five unsolved murders on their hands.

They don't have a single lead.

Then in April, 1970, the Zodiac releases a third code.

- This is spurring his ego.

"Look how intelligent I am. Look how great I am.

I'm gonna give you something

that you are never gonna figure out."

- [Narrator] The latest code contains just 13 characters

and the Zodiac promises that cracking it

will finally reveal his identity,

but no one can crack it because that's impossible.

- This one, he sets up by saying, "My name is,"

and then he delivers 13 characters,

13 symbols that made it essentially impossible to decrypt,

if it wasn't closely related to one of his previous ciphers.

- If the message as short as it is here,

it's not possible to check if your assumption is correct.

- [Narrator] The whole third code is nothing more

than yet another opportunity

to taunt the police for their failure.

- Hundreds of people sat down to try to break it

and came up with plausible sounding explanations,

but pretty much any 13 letter word might be plausible.

There was no way to prove or disprove,

even the likelihood that one explanation

was better than another.

- [Narrator] Something is changing

in the Zodiac Killer's approach.

Killing seems to be taking a backseat

to increasingly bizarre demands

for publicity, and increase in threatening letters.

a diagram of a homemade bomb,

he warns he's going to plant somewhere.

He even demands that the public starts wearing buttons,

carrying his notorious signature symbol.

- These are thoughts of a delusional person.

His narcissism's gone to a different level here.

"I am now impatient.

You need to do what I'm telling you to do.

Why haven't you seen how great

and powerful I am before now?"

- [Narrator] When the demand

that people wear his symbol fails,

the Zodiac sends a fourth code and a map.

He claims that combining the map with a 32-character cipher

will reveal the location of a bomb.

[tense music]

- So the police have this map,

but they need to break this code, if they're going to get,

what seems to be a vital piece of information,

the coordinates where the bomb is hidden

and hopefully save some lives.

- [Narrator] He's no longer boasting about his achievements.

Instead, he's sending a warning,

a warning with just one purpose, fear.

- He's now changed his tactic

and he's doing it to try and build that fear.

He wants the recognition from further afield

and what's more recognizable?

Well, an act terrorism.

- [Narrator] But if the bombs coordinates

really are hidden within the 32-character cipher,

code breakers fail to reveal them,

no explosion is ever reported.

It is the last code the Zodiac ever sends.

[tense music continues]

The letters, however, do keep coming.

Between 1970 and 1974,

up to 10 more are sent to the police and press.

And each time the tally of victims

the Zodiac claims to have killed increases.

- As far as we know, Paul Stine is the last victim,

but another letter is received in which the Zodiac claims

to be responsible for 37 other deaths.

It's an extraordinary claim. Very hard to back up.

[mysterious music]

- [Narrator] In 1974, the taunting letters suddenly stop,

the Zodiac Killer disappears without a trace,

leaving just one last mystery.

Who was the Zodiac Killer?

The police never closed the case.

For decades, the last three codes that could reveal

the Zodiac's true identity, remain un-cracked.

Until 2020, half a century after the killer

sent out his second and most complex cipher.

When a team of amateur code breakers

spread across three continents

attacks the second Zodiac cipher

with cutting-edge software.

- They had tools at their disposal

that just didn't exist in 1970.

By 2020, there was no question

that computers could easily handle running thousands

and thousands of tests of different possible substitutions.

- [Narrator] The team suspects

that after creating his substitution cipher,

the Zodiac may have mixed up the symbols

using a pattern known only to him.

If that was the case,

then before they can even start on his original cipher,

they have to unscramble the symbols.

- If a message in some kind of cipher has been manipulated,

jumbled up in a way that rearranges all the letters,

first, you have to solve that layer.

You have to get back to what the original cipher was.

And then you can go to work trying to break that cipher.

[dramatic music]

- [Narrator] The team tests this theory

by using the message he sent as a starting point,

and trying to work backwards

to reconstruct the killer's original cipher,

hackers call this reverse engineering.

- It's very difficult to decrypt a message

when you don't even know which direction it's written in.

So what they were trying to do essentially

is reverse engineer whatever kind of grid

or algorithm the Zodiac was using

in creating this cipher.

- [Narrator] The team's computer program

generates 650,000 possible versions of the encrypted text.

Any one of them

could be the Zodiac's original ciphered message.

- So there was no guarantee that this would work, right?

It could be that the right permutation was not in their list

because the total number of permutations would be bigger

than the number of atoms in the universe.

- [Narrator] The code breakers run all 650,000 versions

through a decryption program they have created.

Its sole purpose is cracking the Zodiac code.

- In the scramble message, nothing's repeated,

it's all random, but if you can get the randomness

out of it, now you can start looking

for those telltale repeats of certain groups

of characters that could stand in for certain words.

- [Narrator] For weeks, the decryption program

produces streams of garbled text.

Then on December 3rd, 2020 many months into the project,

they make a breakthrough.

- On one of their many versions of the de-scrambled message.

They started seeing some recognizable words.

They saw the phrase "I hope you are."

And then elsewhere on the page, "trying to catch me."

- [Narrator] Painstaking analysis reveals

the successfully decrypted words were read

in a diagonal direction, starting in the upper left corner

and moving across two places for every subsequent line.

- Now they had strong clues

about how the original message was scrambled,

in effect they'd reverse engineered the Zodiac's system.

- So once you know the pattern

for how to reorder the symbols

and you think you've got a lot of the words correct,

you can lock in those words and look for substitutions

for the remaining letters to try

to crack the rest of the cipher.

- [Narrator] The team modifies the decryption software

and runs it again.

And after more than 50 years,

the Zodiac's second code begins to break.

- After going through this process,

the team was faced with an almost complete paragraph.

- The first line reads,

"I hope you are having lots of fun in trying to catch me."

This was a real breakthrough and also kind of chilling,

here were the words of the Zodiac being heard again

for the first time in half a century.

- [Narrator] Mirroring events of more than 50 years ago,

it's amateur code breakers who finally break

into a Zodiac code.

[dramatic music]

Many were hoping that breaking the second code

would finally nail the identity of the man

who terrorized San Francisco half a century ago.

[tense music]

It doesn't, which is no surprise

to those who've studied him.

- This code is repeating the same lines, the same things,

how he enjoys killing.

He doesn't mind dying,

but this doesn't necessarily mean

this is what he's thinking.

This is bravado.

The Zodiac is constantly putting out there

that he's going to reveal himself.

He's gonna tell the police what they want to know.

He's going to come out, but why would he?

The ciphers, the codes that's what's keeping him alive.

If he was a real person, then that would all disappear.

We wouldn't have this like legacy in this myth

that surrounds him.

So he was never going to give anything away.

- [Narrator] The true identity

of the Zodiac Killer remains unknown.

- We still don't know who Zodiac is or was, if he's alive,

if not, if he is, where is he?

That's what keeps this story going,

is the questions that don't get answered.

[eerie music]

- [Narrator] Two of the Zodiac codes remain unbroken

and it's possible they do contain clues

that will one day reveal the identity

of one of history's most notorious serial killers,

but based on the contents of the messages

that have been decrypted so far, that seems unlikely.

- We're still left with the question,

whether his name is really in that code at all,

or is it just another hurdle

for the authorities to jump over?

- I'm sitting here 50 plus years later,

still trying to answer the question.

I think this will just be one of those cases

that we probably will never know.

- Would cracking all of his codes,

give us the final pieces of the puzzle?

Would we know the full extent of his crimes?

And would we understand the mind of that killer?

Perhaps, but that requires us to crack those codes.

- The fact that there are still these un-cracked codes

out there means people will continue

to obsess over the Zodiac,

try to break these codes, try to figure out who he was,

which in kind of a chilling way is probably exactly what

the Zodiac Killer intended.

[dramatic music] [computer beeping]

[adventurous music]

[bright upbeat music]