It was a few days before Christmas in 1983.

13-year-old Sean McCowan woke up to the sight of his mother sitting on his bed.

She was waiting to tell him terrible news.

Sean’s sister, 22-year-old Erin Gilmour, was dead.

Murdered.

Sean was shattered. He would never be the same.

From that moment on, Sean was driven by the question: Who killed his sister?

Investigators would learn that just four months earlier, another Toronto woman had been murdered in a similar fashion.

45-year-old Susan Tice.

Erin and Susan didn’t know each other.

As it turned out, though, they were connected.

The same man had killed them and gotten away with it for 40 years.

But not forever…

Watch the full?episode, “Evil Walked Through the Door” on? Peacock ?now.

You can also watch on the? NBC app ?or listen to it on our? podcast .

After you finish the?episode, you can watch Erin’s loved ones reflect on her life.

You can also watch Susan’s son share some favorite memories of his mother.

Plus, listen to the? Talking?Dateline ?episode?with?Josh Mankiewicz?and Andrea Canning, in which they take you behind-the-scenes of the making of the?episode.

Then follow along on Josh’s travel to the remote northern location of Moosonee, Ontario, where the killer was eventually caught.