For every murderer behind bars there was a mistake that gave them away. Investigators and those close to the case discuss the key error that led to an arrest.
In Elgin, Scotland, a young mother goes missing in 1998. Investigators fear they will never find out what happened until they discover the killer made a key mistake.
In Nottinghamshire, 1983, a 16-year-old girl is found raped and murdered. Decades later, advances in forensic technology enable police to find a DNA match.
In Arizona, teen Ricky Flores goes missing. Police suspect Ricky's friend Willy, but he denies involvement until police expose a racist conspiracy to commit murder.
In Virginia, 2009, student Morgan Harrington vanished. Her body is found, but no culprit is identified until five years later, when another student goes missing.
In Texas, a patrol car policeman is obsessed with being a 'Superman Cop.' But when Susan Harrison spurns his advances, he goes rogue - and she ends up dead.
In East London, a sexual predator found his victims through an online dating app. The victims' families convince the police they are dealing with a serial killer.
In Oxford, a debt-ridden man brutally stabs an antique dealer in a plot to steal a valuable book. A selfie taken by the killer proves a vital piece of evidence.
A young TV actress lived with her drug addicted brother. But when she asks him to leave, he reacts in the most brutal way - with a gruesome plan to...
In the East Midlands, a businessman disappears after a commercial failure. When police find the bodies of his colleagues and their family, they have one suspect.
In Cleveland, a former US marine and convicted sex offender lures vulnerable women back to his home. Few make it out alive.