NEW YORK — A rookie police officer walking with his gun drawn in a darkened stairwell of a crime-ridden public housing complex accidentally shot and killed a man who was leaving the building with his girlfriend, authorities said Friday.

“What happened last night was a very unfortunate tragedy,” Police Commissioner William Bratton said. “The deceased is totally innocent. He just happened to be in the hallway. He was not engaged in any criminal activity.”

The shooting of 28-year-old Akai Gurley, who was unarmed, occurred late Thursday night as 26-year-old Officer Peter Liang and another rookie officer patrolled the Louis Pink Houses in Brooklyn’s gritty East New York neighborhood.

The New York Police Department assigns rookie officers as reinforcements in parts of the city that have seen increases in crime. The housing project, where Gurley’s girlfriend lives, was the scene of a recent shooting, robberies and assaults.

The officers had descended onto an eighth-floor landing when, 14 steps away, Gurley and his girlfriend had opened a door into the seventh-floor landing after giving up their wait for the elevator so he could head to the lobby. The lights were burned out in the stairwell, leaving it “pitch black” and prompting both officers to use flashlights, Bratton said.

Liang, for reasons unclear, also had his gun drawn, police said. He was about 10 feet from Gurley when, without a word and apparently by accident, he fired a shot, police said. In general, officers have discretion on whether to draw their weapons based on what they are encountering or believe they may encounter, Bratton said.

Gurley’s girlfriend, Melissa Butler, told the Daily News she was walking him out of her seventh-floor apartment after braiding his hair when the shooting happened.

“He didn’t do nothing wrong,” Butler said. “He was just standing there, and they shot him.”

She and Gurley made it down two flights of stairs after he was shot, but he collapsed on the fifth-floor landing and lost consciousness, she said.

Gurley was taken to a nearby hospital, where he died, police said.

Gurley’s stepfather, Kenneth Palmer, said officials called his relatives in Jacksonville, Florida, to notify them of the death.

“What’s hitting me on the head right now is how it happened,” Palmer said. “The mood I’m in is pissed off.”

Gurley’s sister and his daughter’s mother, who is not his girlfriend, were to speak Saturday at the Harlem headquarters of the Rev. Al Sharpton’s National Action Network.

Police officials pieced together the details of the shooting from radio reports and interviews with the girlfriend and the second officer, but they said they have not spoken to Liang. The officer must first be interviewed by the district attorney’s office, which will decide whether to file criminal charges, before internal affairs officers can question him, a standard policy.

The shooting comes at a time when the NYPD, the nation’s biggest police department, is changing how rookie officers are used fresh out of the academy to give them more training and time with more senior officers. It also reinforced what some critics say are long-ignored maintenance problems in public housing, including poor lighting.

Public housing employees walked around the buildings Friday with a shopping cart full of light strips and fixed all the lights on the staircase.

The president of the Patrolmen’s Benevolent Association, Patrick J. Lynch, said the complex is among the most dangerous in the city and the stairwells are the most dangerous places in public housing.

State Assemblyman-elect Charles Barron, whose district includes the complex, expressed doubt the shooting was accidental.

“This young man should be alive today,” he said. “This is ridiculous.”

The shooting recalled a 2004 incident in which 19-year-old Timothy Stansbury was shot dead by a startled officer on a Brooklyn rooftop of a housing complex. His family got a $2 million settlement with the city.