1. The Zodiac Killer
The random and brutal murders of five people in California’s Bay Area in 1968 and 1969 and a series of cryptic notes sent by their apparent killer terrorized Northern California for years. The identity of the self-proclaimed “Zodiac Killer” remains a mystery.
2. Chicago Tylenol Murders
In 1982, a series of poisoning deaths caused fear and panic across Chicago. The victims had all taken Tylenol capsules that had been laced with potassium cyanide. The infamous “Tylenol Killer” remains at large.
3. Murder of JonBenét Ramsey
The 6-year-old child beauty pageant queen was found murdered in the basement of the family's Boulder, Colorado, home the day after Christmas 1996. Today, the case remains unsolved.
4. Disappearance of Kyron Horman
The 7-year-old boy went missing on June 4, 2010. He was last seen by his stepmother, who told police she had dropped him off at his school, Skyline Elementary. No charges have been filed against anyone in the case, nor has anyone officially been named a suspect.
5. Disappearance of Madeleine McCann
The 3-year-old vanished while on a family vacation in the Portuguese resort town of Praia da Luz in June 2007. UK police recently announced there are 18 potentially linked cases in the region where the girl disappeared. Those cases involve a male intruder who entered villas where children were staying, officials said. Meanwhile, those investigating Madeleine's case have not given up hope in finding her alive.
6. The Long Island Ripper
Police believe that 10 sets of human remains found in Long Island may be the work of a serial killer. The bodies were found in 2010 and 2011, all dumped along the same desolate stretch of coastline.
7. Jack the Ripper
In 1888, a killer stalked the streets of London's Whitechapel District and committed a series of grisly murders that would add up to history's most infamous serial killer case. More than 125 years later, it remains unsolved.
8. Disappearance of Haleigh Cummings
The 5-year-old little girl was reported missing from her family’s Satsuma, Florida, home on February 10, 2009. Her case remains unsolved and is classified as a homicide.
9. Jimmy Hoffa
The famed Teamsters leader was last seen in 1975 at a restaurant in Bloomfield Township, Michigan. For four decades, police have searched several sites for Hoffa’s remains, but they still don’t know what happened to him or where he may be.
10. The Black Dahlia
Actress Elizabeth Short was found dead -- her body mutilated and naked -- in a vacant lot in Hollywood, California. Although many confessed to the brutal crime, the case remains unsolved.