This doesn't happen in our happy little world.

Their world shatter.

A young mother murdered.

Who could have killed her?

We had a homicide and no suspects.

Also, no evidence.

But police found a dark side behind that bright suburban façade.

She slept with the children and the door locked.

And finally, a vital clue.

It's the most pointed piece of premeditation.

Thanks for joining us.

I'm Lester Holt.

This story involves a young mother who disappeared one morning.

Her husband said she went for a run and never came home.

It was a puzzling case.

It happened in a safe suburban neighborhood.

The kind of place where crime is unexpected.

Even more unexpected, the tiny clue buried in a computer

that unlocked the mystery.

Here's Keith Morrison.

Nothing about that morning made any sense.

It all seemed like just such a bad nightmare.

...