[narrator]
The city's gripped by fear
after a number of horrific and unsolved murders
are committed by a shadowy serial killer.
[dramatic music plays]
The panic reaches a fever pitch
when the killer releases a series of chilling notes
in which he refers to himself...
as the Zodiac.
Three virtually identical handwritten letters
arrived at the offices of three California newspapers.
[eerie music playing]
And those letters took credit for the crimes
and included details from the crimes,
including the ammunition that was used,
the positions of the victims,
and things like that.
And then another letter
that was sent on August 4th of 1969
was the first to use the name...
the Zodiac.
The Zodiac's crime spree is often referred to
as a reign of terror.
And that's because his murders not only sent
fear throughout the community,
but he eventually threatened to murder
schoolchildren on a bus.
He also said he was going to use a bomb.
So there were a lot of threats
that sent fear through the community.
[man 1]
And on top of that,
Zodiac's victims were selected,
presumably, randomly.
And so people felt like, "Well, he can be around the corner
ready to kill me, or us."
So it became kind of like a bogeyman
waiting to kill somebody,
out of the blue.
The Zodiac, as macabre as it sounds,
turned his killings into a game.
Not just for law enforcement to try and figure out,
but for the average citizen.
These ciphers sort of served as a way for people
to play his game.
"Can you figure out who I am?"
So, using these codes,
while it made Zodiac incredibly scary,
it also made him intriguing because...
anybody could figure out
who the Zodiac killer is
if you know how to play his game correctly.
[narrator]
that the ciphers revealed his true identity
taunted the police and the public.
Both the authorities and amateurs alike
pored over the encrypted letters,
hoping to crack the killer's devious codes.
[Jarroush]
it's commonly referred to as the 408 Cipher.
That was actually solved in a matter of days by
a couple of citizens in the San Francisco Bay Area.
[Butterfield]
economics teacher and his wife,
Donald and Bettye Harden,
decided to take the killer's challenge
and spent a weekend trying trying to solve that cipher.
And they were able to find the solution.
Unfortunately, the Hardens' solution
did not reveal the killer's identity.
But it did begin with the phrase,
"I like killing people because it's so much fun."
Then he talked about collecting slaves
for his afterlife.
[ominous music plays]
[narrator]
heightened public panic about the Zodiac.
But the killer's second cipher,
known as Z-340,
would not be solved so easily.
[Butterfield]
consisted of 340 symbols.
Unfortunately, that cipher was very difficult to solve
and he may have made it that way deliberately,
because he may have been upset that his first cipher
was solved so quickly.
[narrator]
the Z-340 Cipher eluded decryption.
By 1974, the Zodiac had released more than 20 letters
in which he claimed to have killed 37 people.
And then, after a five year reign of terror,
his communications abruptly ceased,
and the case went cold.
It seemed that the codes and the crimes
of this evil genius
were destined to remain unsolved.
More than 30 years would pass
before software engineer David Oranchak
became inspired to take on the Z-340 Cipher.
To try and solve the cipher, David Oranchak teamed up
with Australian mathematician Sam Blake
and Belgian computer programmer Jarl Van Eycke.
The team became kind of this collaboration
of people that would run experiments,
try out different ideas,
come up with computer programs
and different kinda specialized tools
in order to try out these ideas
or to just run a bunch of keys and try to break the code.
[narrator]
the Z-340 Cipher seemed like it might be
an unbreakable code.
But then...
after 15 years of challenging work,
the team finally cracked it.
[Oranchak]
to decrypt the message. It was very exciting.
Once we had the solution,
I put together a report and sent it to my contacts
in the code-breaking unit of the FBI.
And then, very quickly they came back to us
and said, "This looks good. It looks strong.
Looks like the real solution."
And that's when the media storm hit.
[narrator]
that the Z-340 Cipher had been broken
more than 50 years after it was released
made headlines around the world.
But unfortunately,
the decrypted message did not reveal
the killer's identity.
However,
the search continues.
Because there are other Zodiac ciphers
that are yet to be solved,
the most fascinating of which
is called Z-13.
This cipher will literally make you pull your hair out.
Because at first glance,
you would think it's got to be easy.
There's only 13 characters in the cipher.
What's worse
is that it starts with the phrase,
"My name is..."
He's literally telling you, "Here's who I am!"
And to this day no one's been able to figure it out
as of yet.