As pen pals go, Washington "Dumplin' " McCaskill has proved an eager and reliable one for state and federal prosecutors.

The burly former enforcer for Central City's notorious "3NG" gang has furnished details on myriad shooters, drug dealers and street beefs across New Orleans. And McCaskill, who has admitted to committing six murders before his 2012 arrest, backed it up with testimony at two major gang trials in the past year. 

But a letter he wrote to an Orleans Parish prosecutor last year has ignited a flurry of legal appeals in one of those cases.

Lawyers for all 10 defendants who were convicted in the federal "39'ers" gang racketeering trial in February are crying foul and asking a federal judge to overturn the guilty verdicts. They claim McCaskill's letter, which turned up after the trial, would have severely tainted the government's two star witnesses.

Those men, Gregory "Rabbit" Stewart and Darryl "Brother" Franklin, offered vivid accounts of street violence, attributing murders, shootings and drug crimes to the accused 39'ers during seven days of testimony.

But in a March 27, 2016, letter to Assistant District Attorney Alex Calenda, McCaskill wrote, "Our federal case is all made up lies. Darryl Franklin and Rabbit lied about a lot of things you think anyone care(?)"

Attorneys for the 10 convicted men claim the feds' failure to disclose that letter before the trial violated Brady v. Maryland, the U.S. Supreme Court ruling that requires prosecutors to turn over all evidence favorable to the defense, including material that could be used to impeach the credibility of a government witness.

"The letter would have been a devastating blow to (the) credibility and veracity of Stewart and Franklin and to the integrity of the government's case based on the credibility of these witnesses," wrote Billy Sothern, the attorney for convicted 39'er Evans Lewis, in a recent motion.

Defense attorneys assailed Stewart and Franklin throughout the six-week trial, alleging they had concocted stories in a bid to shave time off their multiple life prison terms.

Federal prosecutors have yet to respond to the legal challenges. U.S. District Judge Jay Zainey has given them until May 31 to do so. But they are likely to argue that McCaskill's letter offers nothing new that would have changed the jurors' minds.

McCaskill also had claimed in jailhouse phone calls that Franklin and Stewart had lied. He told a woman they had "created the case" and "dug a hole for themselves and pushed everybody else in." Armed with those calls, defense attorneys grilled McCaskill, 38, over those claims during the trial.

He downplayed them, saying he was only referring to "certain things," such as Franklin's purported knowledge of the September 2011 murder of a federal informant, Michael Marshall.

Even so, McCaskill maintained he carried out that hit job with defendants Leroy Price and Ashton Price, earning $7,500 for the hit. He also implicated several other defendants in gun assaults and drug crimes.

But Sothern's motion argues that McCaskill's letter "was unlike any other piece of evidence available to the defense, as it undermined the government’s entire case and star witnesses, and it was written to an Orleans Parish prosecutor."

Just how the government failed to disclose the letter is unclear. Calenda had turned it over eight months earlier to attorneys for 3NG leader Kentrell "Black" Hickerson before his trial last year on racketeering and drug conspiracy counts — at which McCaskill also testified.

Hickerson's attorneys recently passed the letter on to the defense in the 39'ers case.

For his part, McCaskill pleaded guilty two years ago in the 39'ers case, admitting to Marshall's killing and the April 2011 murder of Floyd Moore. He is scheduled to be sentenced June 13 in the federal case, facing a life sentence but hoping for leniency because of his cooperation with prosecutors.

In state court, McCaskill pleaded guilty to three counts of manslaughter in exchange for an 80-year sentence, which would be reduced if Zainey sentences him to less than that in federal court.

McCaskill admitted to killing Lester "Fat Man" Allen in September 2011 and Lester Foster two months later. He also confessed to killing his good friend, Charles "Buck" Anderson, on behalf of the gang.

Anderson was considered the weakest link in a shooting in the B.W. Cooper housing development in December 2011 that left the target, Emmett Allen, paralyzed and 2-year-old Keira Holmes dead, setting off a public outcry.

Anderson was known for his vehicle — he was "Buck with the truck" — and it had been spotted at the killing scene. Police were looking for it, and friendship gave way to gang democracy, McCaskill testified. They voted and decided he had to be killed.

"Everybody thought Buck was going to tell," he said.

McCaskill is due to testify again in June against Kendall Livingston, who is accused of joining him in Anderson's murder.

For all his cooperation, McCaskill acknowledges one blemish on his credibility: He has admitted he lied to an FBI agent when he claimed he bought drugs from Kevin Jackson, a cousin of imprisoned Central City crime boss Telly Hankton.

Follow John Simerman on Twitter, @johnsimerman.