(NBC News) – Tonight on “Dateline,” Lester Holt reports on the murder of four-year-old Barbara Jean Horn, and how the investigation into her death revealed decades of misconduct across Philadelphia’s criminal justice system and raised questions of whether police had caught the right man.

Here’s a preview:

Dozens of detectives searched for leads as Barbara Jean’s murder gripped the city for days, then weeks and the case went cold.

LESTER HOLT: Was it frustration for you and Sharon that, you know, all this time had gone by, and they still couldn’t find the — the perpetrator?

JOHN FAHY: Yes. we were callin’ all the time.

Four years later, Detective Marty Devlin and his partner took over the case. Devlin — who was known as “Detective Perfect” for his uncanny ability to crack the toughest cases — quickly arrived at an old suspicion. This time, about both parents.

SHARON FAHY: They said that I knew that John did it, and that I was protecting him. I said, “I know he didn’t do it. I would’ve already murdered him if I thought he did.”

LESTER HOLT: You were angry.

JOHN FAHY: I was pissed. I was pissed. You know, you would think that, after four years, there was some kind of progress. And there didn’t seem to be any.

But two months later that suddenly changed when the detectives zeroed in on a new suspect. One whose name had never come up before.

LESTER HOLT: When you heard Walter Ogrod, what were you thinkin’?

JOHN FAHY: “Who the hell’s Walter Ogrod?”

“Dateline” Friday’s “The Investigation” is part of the NBC News “Justice for All” series. Watch tonight at 10 p.m. on NBC4.