An exploration of the 1999 attack at a high school in Littleton, Colo., and the myths that may have inspired the active shooter phenomenon.

A white supremacist's deadly attack on a Sikh temple in Oak Creek, Wisconsin and the heroic response by local police, transforms a community.

A shooter's violent path through Santa Monica, Calif., leads to a deadly confrontation on a college campus; an examination of the shooter's mental health.

A community is left devastated and traumatized by the shooting at Pulse Nightclub.

Stories of loss and heroism during the 2013 shooting at the Washington Navy Yard; the shooter's sister recounts her brother's descent into violence.

The city of Charleston, South Carolina is ripped apart by a shooting at a historic church; in the aftermath, the community is divided by the idea of forgiveness.

The 2015 shooting in San Bernardino, Calif. brings out a massive response from law enforcement, while a shocked community responds to homegrown terrorism.

Survivors and first responders recount the 2012 movie theater shooting in Aurora, Colo.; a grieving family challenges the media.

How well do you know your neighbors? What are their names? What do they do for a living? After watching "Nightmare Next Door," you may want to find out a little more about them. Each episode tells the story of a murder that happened in a seemingly tight-knit community. Viewers get to the heart of the case through interviews with investigators, prosecutors, family members and -- of course -- neighbors, in addition to forensic experiments. The stories reveal twists and turns, and when a suspect emerges, the final outcome can surprise everyone involved.

The holiday season between Thanksgiving and New Year's is often thought of as a celebratory period to spend time with friends and family. That is not always the case, though, as the period is not immune from experiencing the same types of violent crimes that occur throughout the year. This series documents stories of tragedy that struck during the holidays, with firsthand accounts from people who had friends or family members killed during the joyous time. Sometimes the victim is killed by someone close to them, such as a case of sibling rivalry turned deadly, but sometimes the killing is done by a stranger who interrupts a holiday gathering with gunfire.

Eagle-eyed technical experts prove there is no such thing as a perfect crime as they assemble the pieces every criminal leaves behind. Dramatic crime re-creations and, sometimes, part of the investigations are a staple of the series. Some of the re-creations include alternate versions of the crimes, which are disproved by science. The show's episodes follow each case from the initial investigation until it reaches its legal resolution.

Is one of America's most prolific uncaught serial killers still alive, and will a new $50,000 reward finally help collar the sadistic criminal? Now known as the Golden State Killer and responsible for at least 12 murders in California, the man investigators have sought for 40 years may be linked to another series of unsolved crimes, raising a key to perhaps solving the case. This five-part docuseries is an in-depth look at the manhunt, patterns, escape methods and mind games of the killer. It also includes interviews with investigators, law enforcement and victims, who offer information to help solve the mystery of a man who has managed to escape justice for decades.