NEW YORK (NBC News) — The murders of two young women near a Phoenix canal in the early 1990s remain unsolved for over 20 years until a team of cold case detectives take on the case. Friday’s all-new “Dateline” examines how investigators used inventive techniques and groundbreaking advancements in forensic genealogy to unmask the killer.

Here is a preview of Keith Morrison’s report:

It was a Sunday evening in November in Phoenix, a wash of cool air, finally, as the sun dropped behind the valley’s mountains. She loved this time alone, as she pedaled the long path by the canal that snaked through the city and out along the valley floor. She could think on her bike, prepare, but of course, we can’t really know what she was thinking, can only imagine, all these years later.

William Hermann: That is burned into my brain. I’ll never ever forget that.

No, not that horror, or the fear that went with it.

Celeste Bentley: I was really scared. I always thought that he was hiding around my apartment, like waiting in a bush to — to kill me.

Jill Kelly: It still gives me goosebumps and a pit in my stomach.

What was it, haunting the Arizona canal?

Rachel Schepemaker: I think it’s just evil and deep and more than we know.

No one knew who did this thing. Was he hiding in plain sight? 

Dateline begins at 9 p.m. Eastern Friday.