Wicked Impulse: In 1997, a California mountain town is stunned by the murder of a longtime resident. Months of dogged investigation bring the case to a shocking conclusion. (S6, ep 1)

Deadly Forest: The tragic story of 17-year-old Michele Avila, who was murdered by two school friends in the Angeles National Forest in 1985. (S5, ep 1)

Pure Evil: When the naked body of a Chicago woman is found dumped on a Wisconsin highway, investigators suspect she may be the victim of a lovers' spat which turned deadly. (S4, ep 1)

Perfect Family: A family are found murdered in their home. Only their 20-year-old daughter has survived. Investigators wonder who would kill this seemingly perfect family. (S3, ep 1)

UFO Surveillance: The team investigates videos indicating a strong UFO surveillance presence at weapons laboratories. (S3, ep 13)

Area 51 Revealed: The team heads to Area 51, where they interrogate former employees of the alleged storage site for alien spacecraft. (S2, ep 13)

First Response: The team attempts to uncover the US military's first responses to UFO encounters, leading them to revisit a suspected UFO crash site in California. (S3, ep 12)

Alien Crashes: In 2008, witnesses saw military helicopters carry away a glowing object. What was the government trying to hide? And could UFO clean-ups be common? (S2, ep 12)

Dark Presence: The team investigates growing numbers of flying orbs sightings, with witnesses also experiencing odd dreams, premonitions and deep fear. (S3, ep 11)

Aliens at the Airport: The team investigate a strange object seen over Chicago's O'Hare Airport in 2006. Are aliens watching our airports? (S2, ep 11)