An ongoing investigation into allegations of police corruption in Brooklyn risks invalidating hundreds of drug arrests made by narcotics officers who were charged with shaking down drug dealers. The exact number of cases that could be overturned will depend on the outcome of a joint investigation between the office of Brooklyn District Attorney Charles Hynes, who recently wrote a gripping novel about police corruption, and the police department’s Internal Affairs Bureau into illegal activity within the Narcotics Bureau Brooklyn South, a command that patrols 13 police precincts in neighborhoods such as Crown Heights, Coney Island, and Bay Ridge.

The five- month investigation has so far netted four drug related arrests of police officers and catalyzed the department to reassign four high-ranking officers and move another 15 officers to desk assignments.

“Anytime you have these kind of things you have to look back over pending cases and probably have to restore some cases that have recently been disposed. We’re not there yet,” Mr. Hynes told The New York Sun. “

The Legal Aid Society of New York, an organization that provides legal assistance to low income individuals, has already begun looking into cases that may have been affected by the four arrested officers.

“At this point we are still doing our research,” a spokeswoman for Legal Aid, Pat Bath, said. “We don’t know how many cases yet.” The investigation into Brooklyn South narcotics began last fall when Detective Sean Johnstone mistakenly recorded himself talking with another officer about underreporting the amount of crack cocaine he had confiscated during a bust. He and a police officer, Julio Alvarez, who was implicated in the recording, were arraigned in Brooklyn Criminal Court in December on charges of official misconduct and falsifying business records.

Following further investigation into the unit, two other Brooklyn South narcotics officers, Sergeant Michael Arenella and Police Officer Jerry Bowens, were arraigned on Saturday on drug charges.

In the wake of the recent arrests, Mr. Hynes’ office made the decision to dismiss several cases in which the charged officers were involved, sources said. A spokesman for the Brooklyn district attorney’s office, Jonah Bruno, would neither confirm nor deny if the office has dismissed or will dismiss any cases involving the officers.

Since drug related convictions often depend on the sworn testimony of arresting officers, and narcotics officers are involved in a high proportion of arrests, hundreds of cases could be overturned, several defense lawyers said.

“Most guilty pleas and convictions related to drug charges are based on the testimony of a police officer,” the executive director of Brooklyn Public Defenders, Lisa Schreibersdorf, said. “Anyone who would shake down drug dealers is capable of committing perjury.” Mr. Hynes, who was a proponent of the 1993 restructuring of the police department’s Internal Affairs Bureau that gave the police a greater ability to recruit high quality officers, is confident that the investigation will yield results.

“The internal affairs bureau is very credible and will run a solid investigation,” he said.

While defense lawyers have already started to pore over cases involving the arrested officers, any chances of overturning cases will likely depend on the officers being convicted, Ms. Schreibersdorf said.

A prominent Brooklyn defense attorney, Kenneth Montgomery, said lawyers defending suspects that were arrested by the officers would be almost obligated to present those facts in court.

Following the arrests of Messrs. Arenella and Bowens, Police Commissioner Raymond Kelly made the decision to transfer the commanding officer of citywide narcotics, Deputy Chief James O’Neill, and three other high ranking officers from Brooklyn South narcotics out of their positions.

Investigators do not believe they were aware of the scandal. A fifth officer, who has not been identified, has been suspended from the force as a result of the investigation.