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.

"Serial Killer with Piers Morgan" delves into the minds of depraved serial killers for the first time. These are individuals who haunted communities for decades, and viewers are given access to maximum security prisons to discover what drove these people to kill. Morgan meets with victims' families and law enforcement as he seeks to understand the actions of these killers and what it took to race against time to stop them. Such interviews include Mark Riebe and Lorenzo Gilyard.

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.

From the outside, families seem like harmonious units in loving homes. But you don't necessarily know what secrets families are hiding from the outside world. This series, narrated by actress Brenda Strong, explores families hiding the most extreme of family secrets: murder. Each episode explores a death in a family, unraveling the motives of the family members, all of whom are potential suspects -- and potential victims. Sibling rivalries, disputes over family businesses and infidelity are some of the factors that lead to the families' ultimate betrayals.

"Betrayed" reveals chilling stories of chasing the American dream and how the veneer can dramatically crumble, ending in murder at the hands of a trusted family member, co-worker, lover, or friend. The cases of deception are told through the lens of the fictionalized, first-person perspectives of the victims. As investigators, family, and friends peel back the layers and facts of each case, the otherworldly narrator already knows where the trail ends. The narrator guides viewers through various relationships, raising suspicions about who will commit the ultimate betrayal.

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.

What lurks in the shadows during graveyard shifts comes to light in this series, a quintessential Investigation Discovery offering that features nail-biting re-enactments of actual crimes in which darkness hides predators hungry for a midnight snack. Told through interviews with victims' family members and law enforcement, each story begins after a brutal crime is committed in the quiet of the night, and community members are left wondering who might be next. Investigators get to work, knowing that each time the sun sets, the body count might rise.