The Brooklyn district attorney’s office has taken a bold step to beef up its sensational case that charges a retired FBI agent, R. Lindley DeVecchio, with aiding a Colombo capo, Gregory Scarpa, in committing four murders between 1984 and 1992, Gang Land has learned.

Sources say prosecutors plan to use a long-imprisoned mobster son of the late Scarpa, Gregory Scarpa Jr., to back up their key prosecution witness — Scarpa’s longtime lover, Linda Schiro — regarding two mob rubouts the son was involved in before he was incarcerated in 1988.

After months of secret talks with the younger Scarpa, prosecutors have decided that his intimate knowledge of his father’s dealings with Mr. DeVecchio far outweighs the heavy baggage that he will carry with him to the witness stand, sources have told Gang Land.

Scarpa Jr. is a multiple murderer with at least half a dozen killings on his extensive mob résumé. Worse, at least as far as being a credible witness goes, he has lied under oath at least twice, at his racketeering and murder trial in 1998, and in 2004 at a post-conviction hearing on behalf of a one-time acting Colombo boss, Victor “Little Vic” Orena.

But the story he has offered to tell is clearly tantalizing to prosecutors: Sources familiar with numerous debriefings say Scarpa Jr. has linked Mr. DeVecchio to the 1984 murder of a former girlfriend of a top Colombo mobster, Mary Bari, 31, and the 1987 gangland-style slaying of mobster Joseph “Joe Brewster” DeDomenico, 44. The younger Scarpa was the triggerman in the DeDomenico killing, and he held Bari down as his father shot her dead in a mob social club, according to court records.

Mr. DeVecchio, 66, is also charged with helping the elder Scarpa orchestrate the 1990 murder of Patrick Porco, 18, then a potential witness against Scarpa and Schiro’s son, Joseph, and with the 1992 murder of a mob rival, Lorenzo Lampasi, during the height of the bloody Colombo war.

The 55-year-old turncoat has told prosecutors for District Attorney Charles Hynes that he helped kill both Bari and DeDomenico after Mr. DeVecchio gave the elder Scarpa negative information about them, sources said.

Bari, a former mistress of a then-fugitive underboss, Alphonse “Allie Boy” Persico, was killed, Scarpa Jr. told authorities, after the elder Scarpa related that Mr. DeVecchio had reported that “she was about to flip,” and Colombo family higher-ups sanctioned the hit. Scarpa Jr. said approval was obtained at several mob meetings, sources said, including one with the gangster for whom he would testify 20 years later, Little Vic Orena.

Scarpa Jr. has told Mr. Hynes’s staffers that his father decided to whack Joe Brewster after Mr. DeVecchio warned him that DeDomenico had become a “born-again Christian and could become a major problem” for the elder Scarpa, sources said.

“He’s risky. I’d keep my eye on him” is what Mr. DeVecchio reportedly told his father, Scarpa Jr. said, according to sources familiar with the turncoat’s account.

In addition to fingering the exagent in the Bari and DeDomenico homicides, sources say Scarpa Jr. also corroborates Schiro’s testimony to the grand jury that his father paid Mr. DeVecchio tens of thousands of dollars in bribes during their long relationship.

He never met Mr. DeVecchio — he was the elder Scarpa’s control agent from 1980 to 1992 — but for several years the younger Scarpa kept a ledger detailing his father’s monthly payments, as high as $2,500, under the heading “miscellaneous,” sources said.

Scarpa Jr.’s current federal prison term for racketeering and murder conspiracy doesn’t end until 2035. While he will receive immunity from prosecution for his testimony, he will not receive any reduction of his sentence for his cooperation with state prosecutors.

In fact, while technically his admissions to committing murders that were part of his federal case should not hinder a pending motion for a new trial, they certainly will not win him any sympathy from the federal judge assigned to his case.

Sources say Scarpa Jr. has admitted lying from the witness stand at his 1998 trial, primarily about his involvement in murders that he blamed on his father. He insists, however, that he testified truthfully when he linked Mr. DeVecchio to payoffs, supplying his father information about mob rivals and other criminal activity, and alerting the younger Scarpa of an impending federal drug arrest in 1987 several weeks after he killed Joe Brewster.

In 1997, when Mr. DeVecchio was granted immunity and forced to testify at a post-conviction hearing for a new trial by Orena, the retired agent angrily denied leaking any FBI secrets to Scarpa Sr. or taking part in any criminal activity with him.

“That’s nonsense,” he snapped. “I did not give him investigative information. I have never given him any investigative information.”

In 1996, two years after three FBI agents reported their belief that Mr. DeVecchio, who was their supervisor, had engaged in numerous improprieties with his top-echelon informer, an internal FBI inquiry cleared him of any wrongdoing. He retired soon after.

In addition to Scarpa Jr. and Schiro, Brooklyn prosecutors plan to elicit trial testimony from the FBI agents who reported Mr. DeVecchio to their bosses, as well as several Colombo family defectors who told the feds after they cooperated that the elder Scarpa had a high-level law enforcement source.

A Brooklyn Supreme Court justice, Gustin Reichbach, has scheduled a trial for September 10.

A month earlier, however, prosecutors must establish at a pretrial hearing that they received no information, directly or indirectly, from Mr. DeVecchio’s immunized testimony to obtain their indictment. The burden will be on the prosecution to show that the grand jury investigation was not tainted by members of Mr. Hynes’s office, or by information that emanated from freelance forensic analysts and journalists whose work triggered the district attorney’s probe in 2005.

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Two weeks ago, the FBI general counsel, Valerie Caproni, was heard apologizing that the FBI had been too aggressive and had overstepped its power to collect telephone records, e-mails, financial data, and other information under the guise of justice.

Two decades ago, however, when Ms. Caproni was an assistant U.S. attorney in Brooklyn, the FBI was anything but aggressive in her view. She was so frustrated by the FBI’s do-nothing approach in finding a murderous federal drug fugitive who’d been indicted in November 1987 that she decided to call in the U.S. Marshals Service to track him down.

And that was before Ms. Caproni began to suspect what she and most authorities now believe: Mr. DeVecchio tipped off the elder Scarpa about his son’s imminent arrest and the dutiful son fled with his wife and 2-year-old son, according to FBI documents.

But a superior advised the young prosecutor to tread softly, noting that the FBI would not be happy if she usurped its jurisdiction in Scarpa Jr.’s case. So in April 1988, she called the FBI’s fugitive squad leader and asked whether “there would be any problem” if she sought help from the U.S. Marshal’s office in Brooklyn. When he told her to “do what you gotta do,” she did just that, according to an internal FBI memo about the episode.

In August 1988, nine months after the Drug Enforcement Administration had officially asked the FBI for help it never received, deputy U.S. marshals located Scarpa Jr. at a Best Western Hotel 70 miles away in Lakewood, N.J., contacted the DEA, and arrested him.

The next day, when FBI officials read about the arrest by deputy U.S. marshals and DEA agents in the New York Daily News, they sent no congratulatory letters to either agency. Instead, they commissioned an inquiry to determine why the U.S. Marshal’s office had not notified the FBI about the impending arrest, as it had done with the DEA.

In explaining the agency’s angst to Ms. Caproni’s supervisor, the FBI agent who conducted the inquiry into the matter stated: “The FBI considers its Fugitive Program to be a significant investigative program.”

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