Story highlights Akai Gurley, 28, was shot after entering a stairwell in Brooklyn with girlfriend

A rookie police officer "accidentally discharged" his weapon, police say

Police Commissioner William Bratton calls victim a "total innocent"

Brooklyn D.A. Ken Thompson says shooting is "deeply troubling"

A rookie police officer shot and killed an unarmed 28-year-old man in an unlit stairwell of a housing project in what New York's top cop said Friday was "a very unfortunate tragedy ... involving an accidental discharge."

The victim, identified as Akai Gurley, was "a total innocent who just happened" to run into Officer Peter Liang in a "pitch black" stairwell at the Louis H. Pink Houses in Brooklyn late Thursday, New York Police Commissioner William Bratton told reporters.

Shot once in the chest, Gurley died at a hospital later.

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

"A life was lost and my heart goes out to the family of the young man," Mayor Bill de Blasio said. "It does appear to have been an accident."

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The shooting comes amid 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 will decide whether to bring charges against the officer.

A few weeks later, the police shooting in Ferguson, Missouri , of unarmed teenager Michael Brown thrust into the forefront the issue of law enforcement's use of deadly force. A grand jury decision on whether to charge Officer Darren Wilson is expected soon.

"What happened in Ferguson is different than what happened on Staten Island is different than what happened in Brooklyn," de Blasio said. "Each of them has their own dynamic."

Brooklyn District Attorney Ken Thompson, in a statement, said the shooting was "deeply troubling and warrants an immediate, fair and thorough investigation."

"Many questions must be answered, including whether, as reported, the lights in the hallway were out for a number of days, and how this tragedy actually occurred," Thompson said.

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 said.

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.

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 said. "It does appear to have been a very tragic accident."