with this letter he made a mistake. he liquid the envelope flap and left enough saliva for dna testing. police now had the dna profile of the zodiac killer. the letter also included another clue. >> i knew it was a code. it was a nine towers because there were nine victims. and i wrote down all the different symbols. and i think i had 23. very close to an alphabet. and then i did something called a frequency count. which is, you write down how many times each letter is used. because for example e is the most commonly used letter in the alphabet. >> karen crawly's father-in-law had been a code breaker in world war ii. together they discovered the alphabet was based on a maritime system of international flags and pendants. and with this discovery, they were able to decipher the zodiac's secret message. >> it said this is the zodiac