In 1997, the leader of sinister cult Heaven's Gate convinced its members to kill themselves in a joint suicide pact.
Responding an an anonymous tip-off, police officers arrived at a mansion in an affluent neighbourhood of San Diego, California.
Inside, cops discovered the bodies of 39 people, men and women, all of whom were found wearing identical clothing.
It emerged the cult's leader had convinced its members to end their own lives, claiming their bodies would transcend to start a new life on board an alien spacecraft.
Leader Marshall Applewhite, a former music teacher, told his believers the spacecraft was hiding behind the Hale Bop comet, which passed earth at the time of the mass suicide.
Applewhite’s body was found among the dead, who all wore badges declaring they were Heaven’s Gateway Club.
One of the first officers at the scene, Detective Rick Scully, said he has vivid memories of the tragedy.
He said: “It was like being in the Twilight Zone. We were wandering from room to room to room, and every room we went into we were finding bodies.
"You're thinking, 'when is this going to end? How many bodies are going to be in here? How many rooms are there to this place?' Because every room we went in had bodies stacked up like cordwood."
Applewhite began to discuss mysticism after he met Bonnie Nettles and opened a book shop and teaching centre to spread their “divine message”.
The pair then went on a tour of the US, and the pair recruited committed followers to their cause in California.
He said anyone who joined his cult would physically ascend to a spaceship, where their bodies would be transformed.
Over the years his belief morphed into the theory the human body is simply a "container" for the soul, and this would be placed in a new body when they reached the spaceship.
In 1985, Applewhite was dealt a devastating blow when Nettles died from cancer, he told his followers that she had transcended to the 'next level' because "she had too much energy to remain on Earth".
It was after Nettles' death that Applewhite really tightened his grip on the cult and he encouraged its members to view him as a Christ-like figure.
Heaven’s Gate started to focus on the suppression of sexual desire with Applewhite and seven others going to extreme lengths and surgically castrating themselves.
Applewhite claimed 'next level' beings had no sexual organs and also forced all the members to wear matching, gender neutral, clothes and to have the same haircuts.
He convinced his followers Nettles was on board a spaceship hiding behind it and this was a sign that she wanted he and the rest of his cult to join her.
On March 26 1997, the group completely cut themselves off from the world and recorded farewell messages.
In Applewhite’s final message, taped on film, he said: "This is your chance. I'm here, I can take you out of here. I can lead you into that kingdom level above human
"That can't happen unless you leave the human world that you're in and come and follow me. Time is short. Last chance.