I'm Lester Holt. Tonight on Dateline, a romantic Valentine's Dance turns riveting murder mystery.

She was just going to Valentine's Dance.

It was after midnight. The front door just pounding.

Pound, pound, pound, pound, pound.

He had blood all over his shirt just screaming. They got her. I was like, who? Who's got her?

This 17-year-old girl that's abducted literally out from underneath the boyfriend's nose.

I was 18. It was just a blind attack.

He'd know me pretty good with the butt of the pistol. Did you recognize this guy?

No. He was dating my sister. My family loved him.

You doubt his story. I had questions. He was at the top of our list. Our primary suspect.

We really had to dig deep.

Someone had thought this out and had planned it.

Are we dealing with a serial offender?

Your heart aches, anger, rage.

We weren't going to let it.

I made a promise to this family.

We're going to keep that promise.

Here's Josh Mancoot's with After the Dance.

Deep inside the Fort Worth Police Department is a room where they keep the cold-case files.

Hundreds and hundreds of them.

Inside each one is a person crying for justice, crying from the grave.

When I walk into this room, that first feeling is like, holy cow, that's a lot of cases. That's a lot of people.

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