I'm Lester Holt, tonight on Dateline, a man who seemed to have it all from money to military metals.

But as dozens of women would learn, he also had a secret.

You were looking for love and decided to try a dating site.

I did. What was his description? He was a military, a Navy SEAL.

He would show up and have flowers, give me a card for no reason. He was a trauma.

My oldest daughter said to me,

either he's the perfect guy or you're gonna end up on Dateline someday.

I googled Derek Allred and I see all his old mug shots.

When I saw the uniform, I knew there was something really wrong because he had a fake badge on.

Every minute he's with you, he is stealing all of your money.

The more we found out about him, it just seemed the more dangerous he became.

He's starting to vanish again.

I had my dogs leashed and next to me, I had a little bagpact for myself.

The second I heard him snoring, I jumped out of my bed.

This guy needs to be stopped.

We all banded together.

He's not gonna do this to another woman.

Here's Andrea Canning with the perfect guy.

It all came down to this moment.

We had officers and agents and vehicles, exit points, entry points to make sure he doesn't escape.

Finally, he was in their sights.

This guy needs to be stopped.

...