New York (CNN) A New York City police officer has been indicted in the death of Akai Gurley , an unarmed man shot and killed in a Brooklyn housing project in November, a law enforcement official with knowledge of the investigation has told CNN.

The charges against Officer Peter Liang were not revealed Tuesday.

Gurley, 28, was "a total innocent who just happened" to run into Liang in a "pitch black" stairwell at the Louis H. Pink Houses in Brooklyn, New York Police Commissioner William Bratton told reporters the day after the killing.

At the time, the NYPD said the fatal shot was an "accidental discharge."

Liang, with less than 18 months on the job and on probationary status, had been placed on modified assignment and stripped of his gun and badge pending an investigation.

"This officer deserves the same due process afforded to anyone involved in the accidental death of another," Patrick Lynch, head of the police union, said in a Tuesday statement. "The fact the he was assigned to patrol one most dangerous housing projects in New York City must be considered among the circumstances of this tragic accident."

Liang's attorney declined comment.

Scott Rynecki, an attorney for Kimberly Ballinger, Gurley's domestic partner and mother of their child, said, "She is at this point feeling that this is the first step in the fight for justice for this wrongful and reckless shooting."

In a statement, Mayor Bill de Blasio said, "It has been reported that a Brooklyn grand jury has acted in this case. No matter the specific charges, this case is an unspeakable tragedy for the Gurley family. We urge everyone to respect the judicial process as it unfolds."

Liang and his partner were part of a "violence reduction overtime detail" at the Pink Houses, where a spate of serious crimes have been reported in recent months, including two robberies and four assaults, Bratton told reporters after the shooting.

The officers had taken an elevator to the building's top floor to check on the roof and were taking the stairs down from the 8th floor, Bratton said, when the officer discharged the weapon. There were no lights in the stairwell leading up to the roof.

Liang drew a flashlight and his weapon "for safety reasons," the police commissioner said. The other officer did not draw his gun.

In the darkened stairwell, Liang's gun discharged about the same time that Gurley, the father of a 2-year-old child, and his girlfriend were entering the seventh-floor landing, Bratton said. Shot once in the chest, Gurley died at a hospital.

The lights on the seventh and eighth floors were not working, Bratton said.

The police commissioner said the decision on when to draw a weapon is the discretion of officers "based on what they are encountering or believe they may encounter. So there is not a specific prohibition against taking a firearm out. But again, as in all cases, an officer would have to justify the circumstances that required him to or resulted in unholstering his firearm."

The shooting also is being investigated by police internal affairs.

"This is a tragic situation," de Blasio told reporters after the shooting. "It does appear to have been a very tragic accident."

The Brooklyn District Attorney's Office had no comment on the indictment, but District Attorney Ken Thompson, in a statement days after the incident, called the shooting "deeply troubling."

Gurley's shooting occurred at a time of strained police/community relations after the July death of Eric Garner at the hands of police on Staten Island. The chokehold death of the unarmed 43-year-old man sparked street protests , a review of police procedures and calls for a federal civil rights investigation. A grand jury declined to prosecute the officer.