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Read more: 鈥�Unbreakable: Eight codes we can鈥檛 crack鈥�

Famous for peace, love, and tie-dye, San Francisco in the late 1960s was also the setting for one of the creepiest real-life murder mysteries in American history. From December 1968 to October 1969, a serial killer using the moniker 鈥淶odiac鈥� murdered at least seven people. The killer taunted local police and newspapers with handwritten letters containing threats, claims of further undiscovered victims, and coded messages.

The supposed killer claimed that the solutions would reveal an identity when they were all solved. Some never were, and the killer was never caught.

The first three codes were encrypted by replacing letters with symbols. But there was a twist: some of the more frequent letters, like 鈥渆鈥�, were assigned to multiple symbols. This made it difficult to solve by simple codebreaking techniques like looking for more commonly used letters.

The three were eventually cracked by supposing that the words 鈥渒ill鈥� or 鈥渒illing鈥� would be in the message. When the results were combined, they formed that described the pleasure the murderer took in killing and described a motivation based on supernatural forces, but no clues as to his or her identity.

In November 1969, Zodiac sent a code to the local papers that law-enforcers still believe could hold the key to solving the case. At 340 characters, it is shorter than the first three codes, and didn鈥檛 use the same encryption method. Dan Olson, chief of the US Federal Bureau of Investigation鈥檚 Cryptanalysis and Racketeering Records Unit, says the code, known as , is number one on his unit鈥檚 internal 鈥渢op 10鈥� list of unsolved codes. He says he gets 20 to 30 submissions from the public every year, but these haven鈥檛 led to any breakthroughs in the case.

鈥淭he encrypted message the killer sent in November 1969 is at the top of the FBI鈥檚 list of unsolved codes鈥�

Bureau cryptanalysts have measured the distribution of symbols in the code to see if it could be shown that a message is indeed present. If there was no solution, or if it was scrambled beyond recognition, then the distribution of the characters in the rows should be equal to the distribution of those in the columns, but that isn鈥檛 the case.

In 2009, Ryan Garlick, a computer scientist at the University of North Texas in Denton, along with his students, tried to 鈥渆volve鈥� a solution using so-called genetic algorithms. They first randomly generated possible pairings of English letters with symbols from the code. They then looked for how well a given set of pairings of Zodiac symbols and English letters produced two and three-letter sequences in the potential solution, like 鈥渢h鈥� and 鈥渆s鈥�, which are common in English.

The best-performing pairings were subjected to further selection and recombination. Using this method, they were able to evolve the solution to the first, solved, Zodiac message, but made little progress with Z-340. Subsequent attempts with genetic algorithms by researchers at San Jose State University in California have yielded similarly disappointing results.

Garlick suspects that some sort of physical rearrangement of the ordering of the symbols is necessary. Testing all the possible ways this could be done, however, is daunting. 鈥淵ou have to happen upon exactly the right thing before any of our software tools would even get close,鈥� he says.

Read more: 鈥�Unbreakable: Eight codes we can鈥檛 crack鈥�