(NBC) — Tonight on “Dateline,” Josh Mankiewicz reports on the story of a young woman who’s shocking murder sparked a national debate on DNA and how it’s collected. But the details of the murder are just as shocking and surprising. This is both a whodunit and the story of what happened next. Mankiewicz warns, “Do not jump to conclusions.”

Our story begins with a break-in — that holds the clue to solving the murder of a lively young graduate student. After hearing someone break into their apartment, two women call 911 and barricade themselves in the bathroom until help arrives. Little do they know, their call for help holds the key to solving the mysterious murder of a young graduate student named Katie Sepich.

The minute her college roommate Tracee Waters called the Sepichs to let them know Katie was missing — Katie’s dad, Dave Sepich, got in the car, bound for Las Cruces to look for his daughter.

“I don’t know where she’s at or what she’s doing, but I’m gonna go give her my piece of mind if I find her,” remembers Dave Sepich.

If he went prepared to give his daughter a lecture, everything changed once he arrived.

“When I walked in the door there was a police captain, and a victim’s advocate, and a minister. And I knew immediately when I saw ’em that it couldn’t be good. And of course, they told me that they had found her body that morning, but they didn’t know who she was ’cause she didn’t have any ID on her. Hit me like a ton of bricks,” Dave Sepich reveals about that night.

Dave Sepich steadied himself and called his wife Jayann. 

Jayann Sepich painfully recalls, “He said, ‘She’s gone,’ and I said, ‘For sure?’ and he said, ‘Yeah,’ he said, ‘I just saw her. She’s dead.’”

It was a seismic shift in the Sepich clan because Katie was full of life from the moment she was born. 

“Little Katie. She was quite something. She was just a ball of fire from day one. Katie was just the most rambunctious little kid you ever met. And she grew up that way. She was just going 90 miles an hour her whole life. She was something,” adds Dave Sepich.

Tonight’s “Dateline NBC” investigation features interviews with the family of Katie Sepich, former Doña Ana County Sheriff’s Department Captain Robert Jones and Las Cruces detective Mark Myers, Katie’s former roommate and more — and explains why Katie’s parents are now determined to change DNA collection laws.

“Dateline” airs tonight on NBC4 at 9 p.m., right after “Lincoln Rhyne: Hunt for the Bone Collector” for another “Crimetime Friday” you won’t want to miss.