the zodiac somehow knew that larry parham was a cancer, a sign different from the other three victims. and the newspapers were calling him a copy cat. >> one zodiac. >> he's angry, he's trying to convince us, no no, i'm zodiac. one zodiac. and, you know, we -- we knew it wasn't him. we knew it was a copy cat. >> to look for forensic evidence one zodiac. and, you know, we -- we knew it wasn't him. we knew it was a copy cat. >> to look for forensic evidence criminalists saturated the letter with ninhydrin, a chemical that reacts with amino acids from skin oils. then the letter was placed in a humidity chamber for several hours. the moisture in the chamber accelerated had chemical process eventually revealing an almost perfect thumbprint. this was photographed and then compared to prints in every fingerprint database available. >> they did databases for city employees, department of motor vehicle, postal workers, including police officers. and nothing. no hits. >> there had been no witnesses. none of the surviving victims could give a description. and the only forensic evidence was a sin