AMNA NAWAZ: For decades, producer Dan Slepian has spearheaded documentaries, podcasts and investigative reports for the news magazine show "Dateline NBC."

In 2002, a chance conversation propelled him to start probing wrongful convictions, work he's continued for more than 20 years.

His new book, "The Sing Sing Files," is out now.

And we sat down recently to talk about why this one issue continues to fuel his work.

DAN SLEPIAN, Author, "The Sing Sing Files: One Journalist, Six Innocent Men, and a Twenty-Year Fight for Justice": You know, everybody thinks they know how the justice system works.

I grew up believing that the justice system worked just the way it should as a kid in Westchester County, a middle-class kid.

And I had a unique perspective from a -- as a "Dateline" producer.

And with that perspective, I was able to see a justice system that I never knew existed, a very dark and ugly underbelly that is really how the system often works.

AMNA NAWAZ: Let me ask you about your entry into that world, because there's this one case in particular, sort of infamous New York case called the Palladium murder back in 1990.

There was a nightclub bouncer who was shot and killed on Thanksgiving Day.

Two men are convicted and sentenced to 25 years to life for that murder.

You called that whole case your window into this world, into the dark side of the justice system.

Why?

DAN SLEPIAN: It was my baptism.

I was out to dinner one night with a detective who I was shadowing a couple of weeks into our project.

And I said: "You must bring this job home with you, everything you see?"

And he says: "I really don't, except this one case has been bothering me."

I'm like: "What's that about?"

And he said that he knew that these two men, David Lemus and Olmedo Hidalgo, were innocent of the murder at the Palladium nightclub on Thanksgiving night 1990.

And I said: "How do you know?"

AMNA NAWAZ: Yes.

DAN SLEPIAN: He goes: "Because I know who really committed the crime."

And so I ended up investigating that case.

I find the real killer.

I get him to Rockefeller Center.

He basically confesses on tape.

The innocent guys spent 15 years in prison.

And eventually after we aired a show back then, the judge overturned the conviction in 2005.

And to this day, not only has the Manhattan district attorney's office never admitted they were wrong,.

They retried the innocent guy.

So, from that perspective, I couldn't believe that people who are guardians of the system, who took an oath to do the right thing, couldn't fix an obvious, tragic miscarriage of justice.

AMNA NAWAZ: And that one case has fueled so many cases you have investigated since then.

You tell the stories of many men in this book, but as you acknowledge, there's one man, J.J. Velazquez, who you say is the soul of this book.

Why is that?

Tell us.

DAN SLEPIAN: He actually shared a wall, a cement wall, he was in the cell next to the guy that I was doing a story about.

And so J.J. wrote me a letter in 2002.

And there was something about J.J.'s letters that really pulled me in.

There was something about his persistence, his eloquence.

AMNA NAWAZ: As a result of your work, years of that kind of work, you get to be there at those moments, like when J.J. is released, right?

And he walks out of prison, and he walks into the arms of his waiting family, his weeping mother.

After the years that you have spent knowing this person is innocent, helping to fight for them to get out, what's that moment like?

DAN SLEPIAN: I obviously was thrilled for J.J. J.J. should never have been arrested 27 years ago.

I had proven his innocence.

He was as innocent as he was alive.

I mean, I did a story about him in 2012, and he spent another decade in prison because of what the Manhattan DA's office did.

So when he got out, I felt joy for him.

But I was not as happy as I thought I was going to be.

And it's not because he wasn't out.

It was because I saw it as something of a failure.

I felt guilt that I knew the truth for so long.

And it speaks volumes about the system that that's what it took.

If it took this to get him out, God help everybody behind him.

It's the tip of the iceberg.

This is a hidden epidemic.

This is a national crisis.

There's only been 3,200, 3,300 people in more than 30 years that have been exonerated in this country.

The most conservative estimates say there's at least 100,000 innocent people right now as you and I are sitting here.

And people go about their life as if it's normal.

AMNA NAWAZ: Sounds like you're still working on these stories.

There are many more to be told.

DAN SLEPIAN: I will never stop working on these stories.

It's embedded in my DNA.

It is my responsibility.

I don't have hobbies.

I don't sleep very much.

I get contacted all of the time by people.

And too many of those claims are true.

AMNA NAWAZ: The stories you document are so powerful.

And I know your work on this continues.

The book is "The Sing Sing Files: One Journalist, Six Innocent Men, and a Twenty-Year Fight for Justice."

The author is Dan Slepian.

Dan, thank you for being here.

DAN SLEPIAN: I so appreciate you having me, Amna.

Thank you.