On the morning of November 13, 1974, thirteen-year-old Allison Louise DeFeo was one of six members of the DeFeo family found shot to death in their Ocean Avenue home in Amityville, New York. Allison was discovered lying face down, shot from behind; the weapon used in the killings was a .35-caliber Marlin rifle. Victims included her parents, Ronald DeFeo Sr. and Louise DeFeo, her older sister Dawn, and her younger brothers Mark and John Matthew.

That evening Ronald DeFeo Jr., age 23, went to a local bar reporting his parents had been shot. Investigators quickly focused on him after inconsistencies in his story and evidence found at the house, including an empty rifle box in his room. He confessed within a day and later said voices had driven him to kill. Charged with six counts of second-degree murder, he was convicted in November 1975 and given six consecutive sentences of 25 years to life.

Accounts DeFeo Jr. gave about the night varied over time, sometimes implicating his sister or unnamed accomplices.