A Suffolk judge is considering whether to throw out a Wheatley Heights man's murder conviction because his defense claims prosecutors withheld evidence that a key witness had been acting as an informant on unrelated drug cases.

Rudolph Bisnauth, now 28, was convicted of second-degree murder three years ago for the June 2009 killing of Jeremiah Armstrong, 47, of Wyandanch. One of the most important witnesses at the trial was Timothy Rich, 35, whom homicide detectives initially considered a suspect.

Bisnauth's attorney, Joseph Ferrante, has argued that state Supreme Court Justice Mark Cohen should throw out Bisnauth's conviction because prosecutors did not notify the trial attorney of Rich's cooperation agreement. Prosecutors are generally required to turn over any evidence favorable to the defense under what is known as the Brady rule.

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Armstrong's murder had its roots in the relationship his daughter had with Rich. They had a child together, but she broke up with him and canceled the insurance on his car after she found out he was engaged and living with another woman, according to court papers. Friends of Armstrong's daughter threatened Rich and he responded by calling her and warning her to get out of the house.

"He explained that his boys were going to 'air out' her house," said Assistant District Attorney Michael Miller in a memorandum of law filed in the case. Prosecutors said Bisnauth and Michael McKenzie, 25, fired about 20 shots at a group of people outside the house on Irving Avenue in Wyandanch on June 1, 2009, killing Armstrong.

McKenzie pleaded guilty to first-degree manslaughter and was sentenced to 25 years in prison. Bisnauth was sentenced to 25 years to life.

Ferrante said if Bisnauth's trial attorney had known about Rich's cooperation agreement with prosecutors, the result likely would have been different.

"Cross-examining a guy with a cooperation agreement is the most devastating cross you can have," Ferrante said. If Bisnauth's trial lawyer had known about the agreement, he said, he could have been able to damage Rich's credibility by showing he would say anything prosecutors wanted to ensure he didn't lose his end of the deal, which got him a 3-year sentence.

Prosecutors say Bisnauth's attorney wasn't entitled to know about the cooperation agreement and, even if he had known, it wouldn't have made a difference in the outcome.

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Miller argued in papers that Rich's agreement wasn't covered by the Brady rule because it was unrelated to the murder charge. Further, Rich got no benefit from prosecutors for his testimony at Bisnauth's trial because the cooperation agreement had already been terminated.

Ferrante said that doesn't matter.

"I should be able to show the jury this is a professional get-out-of-trouble guy," who makes a habit of shortening his prison time by trading information to authorities, Ferrante said. In court papers, Ferrante wrote, "Rather than appearing honest and forthright, he would have been exposed as a lying career criminal who was desperate to evade more prosecutions."

Ferrante noted that after Bisnauth's conviction, Suffolk County Court Judge James F.X. Doyle gave Rich his 3-year sentence in part "because you've been assisting in some other matters the district attorney is interested in."

Cohen is expected to make his ruling in the next few weeks.