When Linda Schiro was just 16 years old, she was already a mistress to the mob. The Brooklyn teen hooked up with an older man, a veteran of the Colombo crime family, and they lived together for nearly three decades.

Gregory Scarpa Sr. was convicted of murder and racketeering and died in a Minnesota prison.

A dozen years later, Schiro has emerged as a key witness against an FBI agent accused in what a prosecutor has labeled one of the worst cases of law enforcement corruption in the nation's history.

The agent, R. Lindley DeVecchio, allegedly fed Schiro's late boyfriend inside information that led to four mob slayings — including hits against another mobster's girlfriend and a gangster turned born-again Christian.

But attorneys for DeVecchio say the longtime mob mistress lacks the credibility to implicate the former FBI agent. Her story has changed drastically over time, the defense says.

Authorities insist that Schiro's initial reluctance to detail the relationship between the agent and the mob capo was motivated by fears for her life. Only recently was she persuaded to tell the truth, they say.

Schiro met Scarpa in 1966 when she was 16. They quickly moved in together and shared the same home for 28 years — except when Scarpa was behind bars. The couple had two children.

DeVecchio was the head of the FBI's Colombo crime family squad and became Scarpa's handler when the mobster turned informer. During Scarpa's time as a mole, DeVecchio put together a 700-page informant file detailing their relationship, court papers showed.

Allegations about possible leaks from DeVecchio to Scarpa first surfaced after the mobster's June 1994 death from AIDS, contracted when he received a tainted blood transfusion. But the Department of Justice declined to prosecute DeVecchio following an internal investigation.

DeVecchio, now 65, retired to Florida in 1996. The former agent has pleaded not guilty to murder charges and is free on $1 million bond.

A law enforcement official, speaking on condition of anonymity because the investigation remains open, said authorities approached Schiro two years ago after a private investigator turned up fresh evidence against DeVecchio.

Prosecutors persuaded Schiro to come clean about the relationship between the agent and Scarpa by promising relocation and police protection. "It was a matter of making her feel safe, telling her, 'You're not going to get whacked,"' the official said.

Court papers filed by the prosecution present a sinister relationship between Scarpa and DeVecchio.

The pair met once a week at Scarpa's home, where the agent accepted a roll of bills bound with a rubber band — the payoff for DeVecchio's tips from inside the FBI, according to an affidavit from Assistant District Attorney Ann Bordley.

The prosecutor maintains that Schiro has direct knowledge about the 1990 slaying of Patrick Porco, who with Scarpa's son Joey and two other suspects murdered a man outside a church in Brooklyn on Halloween 1989.

In May 1990, Schiro answered a phone call from DeVecchio, asking for Scarpa. The mobster had Schiro drive him to a pay phone, where he spoke with DeVecchio for about 10 minutes before returning to the car.

"I can't believe this (expletive) kid," Scarpa allegedly told Schiro. "Patrick is going to rat on Joey. We got to do something about this."

Porco, 18, was found the next morning on a Brooklyn street corner with a bullet in his head.