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As a former Army doctor fights to overturn his 1979 murder conviction for the deaths of his pregnant wife and two daughters, prosecutors are trying to poke holes in his case.

On the third day of an evidentiary hearing before a federal judge, a retired U.S. marshal directly contradicted a former colleague's claims of overhearing a drug user reveal details about the 1970 murders while driving her between detention facilities in upstate South Carolina and Raleigh, N.C.

Former Deputy U.S. Marshal Dennis Meehan said the car ride, described by now-deceased Deputy U.S. Marshal Jimmy Britt, never happened. Instead, Meehan claimed to be the driver who transported Helena Stoeckley to Raleigh and said he picked her up in Charlotte, N.C.

Earlier this week, the defense introduced a sworn affidavit in which Britt described a meeting where he heard a prosecutor threaten Stoeckley with murder charges if she testified that she was at the crime scene during the killings.

But today, prosecution witnesses questioned Britt's credibility. Former Chief Deputy U.S. Marshal Eddie Sigmon said Britt lied about the number of overtime hours he worked and once got into a fist fight with another marshal over a woman.

"I would say he was a very marginal employee," former U.S. Marshal William Berryhill, Jr. said. "I found Jimmy Britt to be fairly large in ego."

Although Stoeckley, who died in 1983, denied having any recollection of the crime when she testified in MacDonald's 1979 trial, her alleged confessions to family and friends have been a central part of the defense's case as they seek to overturn the murder conviction.

Bob Stevenson, the brother of MacDonald's murdered wife Colette told reporters outside the courthouse, "If you take a lie detector test, of what value is it to an insane person talking with a drug-addled mind. It's useless!"

But the woman who married MacDonald while he was in prison said Stoeckley's drug use fit the profile of many people involved in violent crimes.

"(In) all the really horrible, brutal murders, people are strung out on drugs," Kathryn MacDonald said. "I know from [Stoeckley's] statements, she wanted protection. And without protection, she was hedging."

This afternoon, the prosecutor in the 1979 trial testified he never threatened Stoeckley and insisted Britt was never in the meeting room.

"We would not have asked him to be in the room," former U.S. Attorney Jim Blackburn told reporters as he left the courthouse. "It is absolutely untrue. And another word for untrue is a lie."

But MacDonald defended Britt.

"He was a maverick and some people may not have liked that," she said. "I ask you to look up Mr. Britt's record with the U.S. Marshals Service and see how highly he was held in esteem by the judges and his own people and measure that against Jim Blackburn's reputation."

Blackburn's legal career ended in 1993 when he plead guilty to forgery, fraud and embezzlement for faking judges' signatures on phony legal documents and stealing funds from his private law firm to award clients for bogus judgments. After serving three and a half months in prison, he worked as a waiter until he established his new career as a motivational speaker on ethics and law.

The hearing is expected to wrap up Thursday, with prosecutors calling author Joe McGinniss as their last witness. McGinniss wrote 'Fatal Vision' in 1983. The book led to a TV mini-series the following year and was highly influential in the public's perception of MacDonald's conviction.

Whether the story gets a new ending will rest in the hands of the judge.