The FBI's Lin DeVecchio and "The Grim Reaper"

Lin DeVecchio was a highly decorated FBI agent who helped put some of the mafia's most notorious leaders behind bars. He's also one of the few FBI agents ever charged with murder. It was his relationship with a violent mob informant known as "The Grim Reaper" that got him and the FBI in trouble.

He's now telling the story for the first time in a new book and on "60 Minutes." It's rare to get an inside look at the shadowy world of informants and their handlers; it's rarer still to hear an FBI agent describe a brutal mafia killer as his friend. But that is exactly what Lin DeVecchio told us.

He says his informant's information helped cripple the mob. To get it, however, DeVecchio has been accused of making a deal with the devil.

"I didn't make any bargains with the devil," DeVecchio told CNN's Anderson Cooper. "I wasn't dancing with the devil. I was dancing with a guy that was very close to the devil, he was a tough guy. But that was what they were paying me to do. I got close to members of organized crime so we can eliminate them."

Asked if he felt like he walked up to the line, DeVecchio said, "Yeah, sure, I got close to it. I freely admit that."

In the 1980s, the mafia was a big problem in New York City. Surveillance footage was as close as law enforcement often got -- including scenes shot outside a mob hangout called the "Wimpy Boys Social Club" in Bensonhurst, Brooklyn, an area then controlled by the Colombo family.

One of the most notorious wise guys was Greg Scarpa, who had a reputation for brutality.

DeVecchio says Scarpa was "was absolutely fearless. One of the toughest guys I've ever seen."

His toughness may have been legendary but what no one in the Colombo family knew was that Greg ("The Grim Reaper") Scarpa was also a rat. In the 1960s, he had secretly provided the FBI with detailed information about fellow mobsters but he'd had a falling out with the bureau and broke off relations.

In 1980, DeVecchio wanted Scarpa back, so he decided to take a big chance: he drove up to the gangster's home unannounced and blocked Scarpa's car as he was pulling out of his driveway.

"He was like, 'Who the f--- are you?'" DeVecchio remembered. "And I said, 'You know, I'd like to speak with you. I do need some help. You know, I want to get schooled in the life. You know, educate me.' About two weeks later he called. And he said, 'Come alone.' And I did."

FBI agents aren't supposed to meet alone with informants, but DeVecchio got special permission. Scarpa was considered a top echelon informant, and great care was taken to keep their meetings secret.

Extra: A mob murder
Extra: Scarpa's history with the FBI
Extra: Did DeVecchio cross a line?

When law enforcement officials announced one of the most important prosecutions ever against the mob in 1985, very few people knew that Scarpa's information had played a role. It was called the "Commission Case," and it not only catapulted a young prosecutor named Rudy Giuliani into the limelight, but also sent the heads of three of the five crime families away for life.

DeVecchio got promoted and ended up leading two organized crime squads in New York; Scarpa got paid $66,000 for the information he supplied over 12 years, and when he was arrested for credit card fraud, DeVecchio helped keep him out of jail.

"His career and his life and his milieu that he worked in was so different than mine, that it was fascinating to me to learn about that," DeVecchio said. "I liked the guy. You know, I make no bones about that. I'm not ashamed of that. Doesn't mean I condone what he did."

Asked if it was a friendship, DeVecchio acknowledged, "It was a friendship. Absolutely."

Produced by Andy Court and Anya BourgBut Scarpa wasn't just a run-of-the-mill mobster. He was a stone-cold killer. He is believed to have murdered at least 12 people during the time he was an informant.

DeVecchio told us that on one occasion Scarpa even let him know he had killed another mobster.

"There was a body found in Brooklyn one day. I said, 'Who did the work?'" DeVecchio remembered.

By "who did the work?" DeVecchio says he meant who committed the murder. "And, he smiled," DeVecchio recalled.

Asked what that smile told him, DeVecchio said, "That smile told me he did it. He didn't tell me he did it. But, I knew him well enough that, you know, he gave me that knowing look, like, 'I did it. You know I did it. And, I'm telling you I did it. But I'm not telling you I did it in words.' "

"And when he smiled, and you thought, 'Okay, he killed that guy,' is that something you would investigate?" Cooper asked.

"No. No," DeVecchio said. "The guy was dead. I mean, as callous as that may sound, the person was dead. It's not gonna bring him back."

"You are absolutely not supposed to keep an informant on the street that is killing people," Ellen Corcella, who was a federal prosecutor who worked on cases with DeVecchio's squad, told Cooper.

"Even if that person is giving you valuable information which may have other arrests as a result of it?" Cooper asked.

"And the question I would put to your question is, 'When is it valuable enough information that you let people continue to kill other people on the street?'" Corcella replied.

In 1991, a war started between two rival factions of the Colombo family. Scarpa is believed to have killed more people than anybody else.

"The whole time you're meeting with him during the war as an informant, he's also going around killing people," Cooper pointed out.

"Yes, he is. That's correct," DeVecchio replied.

"And did you know that at the time?" Cooper asked.

"Sure. I knew that because I knew the guy he was," DeVecchio said.

"But, if you know the guy you're talking to to stop the war is actually the guy who's conducting the war...," Cooper remarked.

"He wasn't the only one conducting the war. Everybody was shooting everybody," DeVecchio said.

"I think that it became more valuable to him to have this informant on the street than it was to pursue other law enforcement goals," Ellen Corcella said.

Asked if she thinks DeVecchio was protecting Scarpa, Corcella said, "Yes."

Protecting him, she says, by giving Scarpa sensitive law enforcement information. Prosecutors first became suspicious when Scarpa's son, who was also involved in organized crime, went on the run just before he was going to be arrested. Those suspicions grew when some of Scarpa's crew got busted and started talking to authorities.

"There was at least two of them that told us they had a source in law enforcement. And one of them basically was the guy who stayed at Scarpa's right hand," Corcella said. "So he saw Scarpa go off and get phone calls, and then come back and say, 'Here's an address where I think we can find somebody to kill. I just got it from my source.'"

After hearing what Scarpa's associates had to say, four FBI agents accused their boss, Lin DeVecchio, of leaking information to Scarpa.

One of the agents specifically remembered DeVecchio asking for addresses that ended up in Scarpa's hands, and these were no ordinary addresses: they were the suspected hide-outs of mobsters Scarpa was trying to kill.

"Did you ever give Scarpa any law enforcement information?" Cooper asked.

"Never, never," DeVecchio insisted.

"His son disappeared right on the eve of him about to be arrested by the DEA. Did you tip him off for that?" Cooper asked.

"No, I did not," DeVecchio said. "I went to him, and asked him, I said, 'You know where he is?' He said, 'I don't know where he is, Lin.' Was he lying to me? Maybe. But, he looked me in the eye and said, 'I don't know where he is, Lin.'"

The FBI launched an internal investigation of DeVecchio, which went on for years but came up with nothing. In 2006, however, the Brooklyn district attorney charged DeVecchio with murder, accusing him of telling Scarpa who to kill and where to find them.

The Brooklyn district attorney's case fell apart when a key witness was thoroughly discredited during the trial.

Former FBI agents who supported DeVecchio applauded as the charges were dismissed.

But the judge who released DeVecchio criticized him and the FBI for making a "deal with the devil" - letting Scarpa get away with his crimes for close to 15 years.

DeVecchio retired from the FBI in 1996 and now runs a private investigations business in Florida. He feels he has been wrongly accused and unfairly maligned, which is why he has written a book and is speaking to "60 Minutes."

"We effectively broke the back of organized crime by using people such as Gregory Scarpa," DeVecchio told Cooper.

"You basically gave this guy a pass because he was giving you good information," Cooper remarked.

"If that's the perception that people have, so be it. I can't change their mind," he replied.

Asked if that is the truth, DeVecchio said, "No, I don't think it's the truth at all."

"You kept him out on the streets, a guy who you knew, at the time, was murdering people," Cooper said.

"It's wonderful to sit in your ivory tower somewhere, and say, 'Oh, how can you do that?' Well, why don't you go out in the street and try working that some time? Try makin' a case. Try talkin' to a wise guy. Try getting that information," DeVecchio replied.