You never forget the sight of a 16-year-old body lying on her back on the riverbank with a ligature around her neck.

This is being described as one of the nastiest homicides in King County record.

25 detectives have been assigned to the case full-time, making this one of the largest

investigations in the county's history.

Seven women have been found dead, and another seven are missing.

King County police say it's reasonable to assume other bodies may be out here.

In the summer of 1982, a body washed up on the shores of Green River near Seattle.

It was by no means a common occurrence, but the site was close enough to a big city, so it wasn't unheard of either.

Investigators identified the body as Deborah Lynn Bonner and got to work on closing her case.

Then, two days later, a river rafter reported two bodies just upstream from where Deborah was found.

They were later identified as Marsha Fay Chapman and a few feet away, 17-year-old Cynthia Jean Hines.

Then, the lead detective on the case, David Reichert, found something they didn't expect.

As I processed that scene, I found another body.

We'd come across Opal Mills.

She's a 16-year-old girl laying on her stomach, a large bruise on her buttocks,

ligature around her neck laying face down, and, you know, we just started doing our job.

One thing became very clear that day.

Investigators were searching for a serial killer, and they would spend the next 20 years searching,

with a body count climbing to nearly 50 before the murderer was caught, making the Green River

killer, the single most prolific serial

killer in U.S. history.

One third of all murder cases in America remain open.

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