(NBC News) – Tonight on “Dateline,” when a complicated love triangle ends in murder a pink gun leads to an arrest until new evidence prompts investigators to rethink the case.

Here’s a preview of Josh Mankiewicz’s report:

It began with a letter mailed from a sun-scorched stack of cement blocks and steel wire on the dusty plains outside Amarillo, Texas: Randall County Jail.

We often hear from inmates. But this letter seemed a little different.

Within the neat lines, the author dangled a promise: “I will give you one hell of a story…”

Now, that was the truth.

The tempestuous tale that unfolded was about two family-oriented women, Robin and Katie, friends from church who fell hard for the same guy and then somehow turned on one another with a vengeance.

ERIN HOUSE: Once she suspected something, Katie’s claws came out.

The dizzying spiral of betrayal and bad blood between the women exploded into violence, ending in a merciless night-time murder, and years of lies and suspicions.

ERIN HOUSE: “Don’t be alone with her. She’s the kinda crazy that’ll kill ya.”

Keep this in mind, the letter made one thing very clear, nothing was quite as it seemed.

You can watch Friday’s “Dateline: The Pink Gun Mystery” tonight at 9 p.m. on NBC4.

About ‘Dateline’

“Dateline NBC” is the longest-running series in NBC primetime history and is in its 33rd season. Dateline is anchored by Lester Holt and features correspondents Blayne Alexander, Andrea Canning, Josh Mankiewicz, Keith Morrison and Dennis Murphy.

The stories range from compelling mysteries to powerful documentaries and in-depth investigations. When major news breaks, they go to the scene, putting the pieces together to bring the viewer the full picture. And in every story they tell, they help the real people who lived the events share their journeys with the viewer.