Julie Williams, Lollie Winans, Paul Fugate and others are among those who have been killed in national parks and whose cases have yet to be solved.

True-crime writer Billy Jensen empowers readers to become citizen detectives, using social-media resources to identify uncaught criminals. He explains how he does this without interfering in police investigations and tells us about his book, 'Chase Darkness With Me.'

At 89-years-old, David Carpenter—the 1980s serial killer known as the 'Trailside Killer'—is the oldest death row inmate in California. We explore Carpenter's early life, crimes and trial.

There was nothing on the surface that raised red flags about Robert Hansen. The unassuming Anchorage native was married with children and was the successful owner of a bakery. But Hansen eventually confessed to killing 17 women in the Alaskan wilderness in a most heinous way.

Ken Rex McElroy, a bully who menaced the farming community of Skidmore, Missouri for years, was shot and killed on July 10, 1981 outside a local bar. Witnessing the murder were McElroy's wife and dozens of local residents. Yet the gunman has never been identified.

It's been nearly a decade since four bodies were discovered spread over a quarter of a mile stretch of beach in Long Island, New York. Since then, other remains have been found, with 10 victims or more now likely tied to a single perpetrator.

Israel Keyes—a murderer who perplexed even the FBI's top minds in criminal profiling—traveled America leaving 'kill kits' with weapons and other items in secluded locations, picking them up before abducting and murdering a chosen victim.

In 1978, four young people disappeared during their shift at a burger restaurant in Speedway, Indiana and turned up dead two days later. What happened remains a mystery and the case has yet to be solved.

How did Charles Manson, an illiterate ex-con, become such an influential cult leader and turn a group of peaceful hippies into cold-blooded killers? Journalist Tom O'Neill set out to answer this and more in his book, 'Chaos.' Read about Manson's early years—starting with his neglectful mother, then moving on to his stints in institutions for 'delinquent' children and ultimately, in federal prison.

During the 1970s, David Berkowitz, also known as 'The Son of Sam,' prowled New York City, fatally shooting couples in cars. By the time he was arrested in August 1977, he had killed six people and wounded seven others. Although the murders stopped after his arrest, several people who worked on the Son of Sam case are convinced Berkowitz didn't act alone.