1. Jack The Ripper

1888 was a bad year to be a prostitute. Between August 7 and November 10 of that year, five women were killed in the Whitechapel district of London’s East End, their throats slashed and their bodies mutilated in a way that indicated they all met their fates at the hands of the same person. One victim’s kidney was even mailed to the police, along with a series of taunting notes penned by someone calling himself Jack the Ripper. Serial murder was a relatively new phenomenon and the attacks were highly publicized. The law’s failure to identify the killer led to such an outcry that both the home secretary and London police commissioner resigned in disgrace.

Jack the Ripper, whoever he was, has been the subject of hundreds of books and articles. The theories surrounding his identity vary from a covert Masonic plot to a member of the royal family. Here are the most likely suspects:

Montague Druitt, a barrister with knowledge of human anatomy. Rumored to be insane, he disappeared after the last murder; his body was later found floating in the River Thames.

George Chapman, a barber who lived in Whitechapel during the time of the murders and who was later found guilty of poisoning three of his wives.

Aaron Kosminski, a Whitechapel resident known for his affinity for prostitutes. He was hospitalized in an asylum several months after the last murder.

2. The Zodiac Killings

So began one of the many encrypted letters sent to San Francisco newspapers by the man who called himself the Zodiac. For most of 1969, a serial killer terrorized Bay Area residents, killing five and possibly more. It started on Dec. 20, 1968, when a couple was shot to death while sitting in a car on a lover’s lane. The killer would strike several more times over the next 10 months, shooting a couple in a public park, trussing up and stabbing yet another man and woman near a peaceful lake, and shooting a cabdriver in the head.

What made the case so fascinating, though, was the way he toyed with police and reporters. He called in several of the murders and began to send coded letters to newspapers, using a cross within a circle as his symbol. At one point, he mailed in a piece of bloodied shirt to prove he was who he claimed to be. Another time, he threatened to shoot up a school bus full of children. The investigation went on for years. Several suspects were considered and questioned, but to no avail. The Zodiac was never caught.