(NBC News) — On “Dateline,” Larene Austin discovers a murder scene in her friend’s garage and investigators believe whoever killed LaNell Barsock had known her personally.

Josh Mankiewicz offers a preview of Friday’s all-new episode, “The Breakup”:

The sun was setting as a woman rushed into the Palmdale sheriff’s station. Her clothes — covered in blood.

Larene Austin said the horror began when she’d walked barefoot into her friend’s garage and slipped on something wet. A terrible scene began to come into focus.

Detectives Joe Espino and Bob Kenney went to the crime scene trying to piece together the short life and brutal death of nurse LaNell Barsock.

JOSH MANKIEWICZ: She had a black plastic bag over her head?

DETECTIVE JOE ESPINO: Yes.

DETECTIVE BOBBY KENNEY: Usually, it means they don’t want to see that person. So, we made the assumption that whoever had done this killing knew the victim.

It was an unthinkable end for a woman devoted to helping others.

Investigators discovered a plot so sinister — so stunning, it took even seasoned detectives by surprise.

DETECTIVE BOBBY KENNEY: The lights start to go on that we’re heading in the wrong direction.

Watch “Dateline: The Breakup” Friday on NBC4 at 10 p.m.