(NBC News) — On Friday’s “Dateline,” lead detectives speak out about the investigation into once-prominent South Carolina attorney Alex Murdaugh, convicted last March of murdering his wife Maggie and son Paul.

For the first time, South Carolina Law Enforcement Division investigators Britt Dove and David Owen speak out about the moment they discovered video evidence proving Murdaugh was at the scene of the murders when they occurred. 

NBC’s Craig Melvin takes you inside the investigation of the Murdaugh murders. Here is a preview of his report:

For more than two years now, the Alex Murdaugh case has dominated the headlines. You may think you’ve seen and heard it all. But you’ve never seen this.

For the first time, the team of investigators that cracked the case speaks out. Telling details only they knew. You’ll hear about their dramatic discoveries.

ALAN WILSON: I knew. When I heard his voice, I knew.

DAVID OWEN: I was really excited.

CRAIG MELVIN: Why?

DAVID OWEN: Because I can prove that Alex was lying to me.

And the moments that changed the case.

ROGAN: I– I’m, I’m 99% sure that was Mr. Alex talking.

RYAN KELLY: We all keep pocketknives. So, you pull out a pocketknife and you kinda — bend down and — and we look at the puncture and we put our knife up to it and it just seems to match.

The moments that nearly broke them.

DAVID OWEN: My mother passed away the day I was set to testify. I took that afternoon to collect myself and think about how we’re gonna proceed.

And their cat-and-mouse game with the most daunting adversary they’ve ever faced.

AGENT OWEN: Did you kill Maggie?

ALEX MURDAUGH: No. Did I kill my wife?

AGENT OWEN: Yes sir.

ALEX MURDAUGH: No, David.

DAVID OWEN: That response struck me.  It was like, wow.

RYAN KELLY: If he was willing to steal from those people, there’s no telling what he would do to his own family.

With never-before-seen video of Alex Murdaugh’s brothers.

RANDY MURDAUGH: Totally out of control.

And his son, Buster.

BUSTER MURDAUGH: Something terrible has happened. Someone was probably trying to kill my brother, that my mom was probably just there.

Plus, the Murdaugh family housekeeper, who saw and heard things no one else did.

BLANCA TURRUBIATE-SIMPSON: She really did not want her family to know that a lotta people had turned on her.

The story you really don’t know — from the only people who can tell it.

CRAIG MELVIN: Did you know in that moment that the case had been blown wide open?

DAVID OWEN: Oh, yes.

A “Dateline” exclusive, Friday at 9 p.m. on NBC4.