Season 4

Examining how Howell Donaldson III terrorised a neighbourhood in Tampa, Florida in 2017, killing four random strangers.

After Gordon Wardell makes an emotional appeal to find his wife's killers, police soon realise they must look closer to home.

Opportunity For Riches - Steven Sidebottom: Initially deemed accidental, handyman Brian McKandie's death was soon considered murder due to his severe injuries.

The Highway Hunter - Adam Leroy Lane: When police find the body of Monica Massaro in her home with 17 stab wounds, they're led to serial killer Adam Leroy Lane.

When Lanell Barsock is found dead in her home in Palmdale, California, evidence points to her boyfriend before investigating detectives start to focus on her best friend, Larene Austin.

Although 17 years had passed since his wife died in a car accident, Malcolm Webster found himself becoming the focus of Scotland's longest-ever criminal trial when police re-examined her death.

When Julie Jensen is found dead, police suspect her husband Mark of murder - despite the seemingly happy family life.

When elite racer Anna Wilson was shot and killed, police launched a manhunt to Costa Rica in search of Kaitlin Armstrong.

Benjamin Laing: Six days after Matthew Manwaring and his daughter Alison vanish, police uncover a grisly scene at Benjamin Laing's home.

Douglas Ward: When young kickboxing champion Niall Dorr was beaten to death in Dundalk, CCTV helped detectives convict his killer.