Investigators said Diane McDermott was fatally shot by her then-boyfriend John Sponza, who allegedly had connections to organized crime.
At the time of her death, police categorized the gunshot wound that killed her as self-inflicted.
The Republican-American, which first reported the most recent findings, said police reopened the investigation last year because the actor was not satisfied with the initial conclusion and sought answers they couldn't provide.
E! News said Waterbury police say they have since gathered enough evidence, including ballistics reports and testimony provided by Dylan McDermott, to prove Sponza was his mother's killer. Sponza was shot to death in 1972.
Dylan McDermott's sister, Robin Herrera, told the Republican-American she is relieved the case is now closed.
"I'm happy to know my mother wasn't mentally ill or depressed," she said. "Somebody took her from us; she didn't leave us."
Dylan McDermott is best known for his work on TV's "The Practice" and "American Horror Story."