In the midst of an investigation into police corruption in Brooklyn, the Brooklyn District Attorney’s office has made the decision to overturn drug convictions against three men who were on the verge of going to prison based on the testimony of two allegedly corrupt police officers.

Albert Mitchel, 43, who was set to serve nine months in prison based on testimony given by a member of the Brooklyn South narcotics squad, Sergeant Michael Arenella, will have his case dismissed today, Brooklyn District Attorney Charles Hynes confirmed yesterday.

Two other men, Jerome Cruz and Lashane Westbrook, who were both found guilty on drug charges in October, had their convictions overturned about a month later following the arrest of another Brooklyn South narcotics officer, Detective Sean Johnstone, Mr. Hynes confirmed. Mr. Hynes’s office has so far dismissed 161 pending cases, which include 129 misdemeanors and 31 felonies, as a result of the investigation into corruption within the Brooklyn South narcotics unit, he said. Narcotics Bureau Brooklyn South is a command that patrols 13 police precincts in neighborhoods such as Crown Heights, Coney Island, and Bay Ridge.

Mr. Mitchel, whose charges will be dismissed today, was arrested last year on October 4 at about 1 a.m. for allegedly possessing 17 plastic bags of crack cocaine, according to a complaint filed in Brooklyn Criminal Court.

In the following months, Mr. Mitchel returned to court four times, where on at least one of those occasions Mr. Arenella testified to his guilt. Facing a hefty sentence, he pled guilty to possession of a controlled substance in exchange for nine months in jail; a term that would likely have begun after his sentencing scheduled on February 13.

But as the probe into Brooklyn South narcotics dug deeper, the Brooklyn District Attorney’s office decided to advance Mr. Mitchel’s sentencing, and he will be cleared of the charges today, officials confirmed.

The investigation into the 260-member Brooklyn South narcotic squad began about five-months ago when a supervisor of Mr. Johnstone’s discovered a recording of the detective boasting that he and a police officer, Julio Alvarez, had underreported the amount of drugs they had confiscated during an arrest. Both officers were arraigned on fraud charges in December.

As the Brooklyn district attorney’s office probed Mr. Johnstone’s past, it was found that two men arrested in 2005, Mr. Cruz, who was facing 3.5 years in prison for selling crack cocaine, and Mr. Westbrook, who was facing a year in prison for drug possession, had been wrongfully found guilty based on his testimony. Both men’s sentencing dates were advanced and their convictions were overturned in November.

Mr. Hynes, who investigated police corruption as chief of the Rackets Bureau in the 1970s and who is the author of a riveting novel on police corruption, said he has full confidence that both his office and the Internal Affairs Bureau will get to the bottom of the scandal.

“I’ve been around a long time,” he said. “The internal affairs bureau I knew as chief of rackets bureau can’t even compare with the respectable one created by Commissioner Kelly.”

The police department shook up the narcotics bureau this week following the arrests of Mr. Arenella and a police officer, Jerry Bowens, who were caught in a sting operation. Four high-ranking officers in the narcotics bureau were transferred out of their positions, and at least 15 members of the Brooklyn South unit have been assigned to desk duties.

On Wednesday, in the latest disciplinary action resulting from the investigation, three narcotics officers were suspended from the force for not cooperating with the Internal Affairs Bureau, a law enforcement official said.