Collier (Boyle) Landry testified in 1990 that he heard a scream and two loud “thuds” on the night his mother, Noreen Boyle, went missing in late December 1989. The then 12-year-old’s testimony helped to convict his father, Dr. John Boyle, of murder. Boyle is currently serving a life sentence at Marion Correctional Institution in Ohio.

TELL DR. PHIL YOUR STORY: Are you involved in a story that has made the headlines?

In the Investigation Discovery documentary, “A Murder in Mansfield," authorities say there may have been another witness to the events of that night. Collier’s younger sister, Elizabeth, who had recently been adopted by the Boyles, may have seen what happened.

“I don’t see how she couldn’t,” says Collier, adding, “Every single night she would go to bed in her room, which was right next to my mother’s room, and she would end up in the bed with my mother the next morning.”

WATCH: Man Who Testified Against Dad In Mother's Murder Trial Claims, 'It Was Completely Premeditated'

Collier says when he woke up the morning after hearing the disturbance, he found Elizabeth in their mother’s bed.

In the video above from Wednesday's exclusive interview, Collier, now 40, tells Dr. Phil that after Noreen’s murder, Elizabeth was adopted by another family, and he hasn’t seen her in over 28 years.

Check here to find out where you can watch.

Footage/photos provided by Investigation Discovery. “A Murder in Mansfield" premieres on Investigation Discovery and IDGO at 9 p.m. EST on Saturday, November 17, 2018.

TELL DR. PHIL YOUR STORY: Have a story Dr. Phil won't believe?

Today’s Takeaway: The Domino Effect Of Trauma