The Republican-American newspaper reports Waterbury police reopened the investigation last year into Diane McDermott's death after Dylan McDermott contacted them with questions.
He was 5 when his mother was shot in February 1967. Her death was originally ruled an accident.
Police tell the newspaper that the evidence they found would be enough to file murder charges against John Sponza. But Sponza was killed in 1972, his body found in the trunk of a car in a Massachusetts grocery store parking lot.
Dylan McDermott declined to comment to the newspaper about the investigation.
He won a Golden Globe in 1999 for his role on the TV drama "The Practice."