Diane McDermott was shot dead in Connecticut in February, 1967 - when the American Horror Story star was just five years old. Her death was originally ruled an accident.
Police officers in Waterbury reopened the case last year (11) and launched a new investigation after the actor approached detectives with lingering questions and they've now determined Diane was killed by her gangster boyfriend John Sponza, reports local newspaper Republican-American.
Sponza was found dead in the trunk of a car in a Massachusetts grocery store parking lot in 1972.
The actor has not yet commented on the new findings, but his sister, Robin Herrera, has since spoken out to express her relief at learning the truth, telling Fox News, "I'm happy to know my mother wasn't mentally ill or depressed. Somebody took her from us; she didn't leave us."