Akai Gurley was accidentally shot and killed by a police officer in a Brooklyn building, cops said.

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A panicked rookie cop in a pitch-black housing project stairwell killed an unarmed man with a single gunshot to the chest as the officer fumbled around in the darkness with a flashlight and a handgun.

The fatal shooting, which officials described as a tragic accident, happened during a vertical patrol late Thursday — months after the superintendent of the Brooklyn development asked NYCHA to fix the stairwell lights.

But it wasn’t until Friday morning — hours after Akai Gurley was killed — that workers finally replaced the lights at the Pink Houses in Brooklyn.

The helpless girlfriend of 28-year-old Gurley, 28, recounted watching him die in a puddle of blood after the routine police patrol turned into a horror show.

“They need to pay for his funeral,” said Melissa Butler, 27. “They killed him. Ya’ll killed an innocent man.”

Gurley was gunned down when probationary Officer Peter Liang’s .9 mm accidentally discharged, with a bullet ricocheting off the wall and tearing into his chest, sources said.

A devastated Liang “was a crying mess,” a police source told the Daily News. “Breathing heavy. He was sitting on the floor for 45 minutes. He was a mess.”

His partner, in a version of Thursday night’s events echoed by Mayor de Blasio and NYPD Commissioner Bill Bratton, said the fatal shooting was a bizarre mistake.

Gurley was shot by a police officer in the stairwell of a building in the Louis Pink Houses public housing complex.(JULIO CORTEZ/AP)

Liang clutched his Glock .9 mm in his left hand and a flashlight in his right when he entered the stairwell around 11:15 p.m. from the eighth floor.

Liang “heard a noise,” the police source told the News. “It was dark. He must have been nervous.”

Gurley and Butler, who had just finished braiding her boyfriend’s hair, only took the stairs because of a slow-moving elevator. They opened the seventh-floor stairwell door at about the same instant that Liang and his partner entered from one floor up.

Liang was upstairs from Gurley when his gun accidentally fired as he opened the door, his partner Officer Shaun Landau told investigators.

But it was unclear if the left-handed Liang was holding the weapon in the same hand that he used to open the door. A mark on the stairwell wall indicates the bullet ricocheted before striking Gurley. it was the only bullet fired.

The wounded man actually ran for his life after hearing the gunshot, and didn’t realize he was bleeding until collapsing on the fifth floor.

“I shot him accidentally,” the devastated cop confessed to his colleagues. His partner Landau never pulled his weapon from its holster.

Bratton and de Blasio agreed the shooting was a terrible mistake — although Liang had yet to tell his story to police.

“What happened last night was a very unfortunate tragedy,” Bratton said Friday. “... It appears this may have been in fact an accidental discharge.”

Bratton, along with de Blasio and First Lady Chirlane McCray, visited Gurley’s family in Red Hook, Brooklyn, for about 10 minutes on Friday night. They didn’t speak to reporters, but Kirsten Foy, of the National Action Network, was inside the apartment during the meeting.

Foy said the city’s top cop, the mayor and McCray hugged and kissed Kimberly Michelle Ballinger, a woman identified as Gurley’s “common-law wife” and the mother of his 2-year-old daughter.

“They offered their apologies, condolences and support going forward so she was heartened by that...grateful,” Foy said. “It was a very human moment for them.”

Devastated gal pal Butler, with tears pouring down her face earlier Friday, said Liang blasted Gurley without identifying himself or even addressing the couple.

The only sound was the deafening echo of the gunshot in the stairwell at the Pink Houses.

Kimberly Ballinger, "common-law wife" of Akai Gurley, met with Mayor de Blasio, Chirlane McCray and Bill Bratton.

 (JOE MARINO/NEW YORK DAILY NEWS)

“They didn’t give no explanation. They didn’t identify themselves,” said Butler, who began dating Gurley in January 2011. “No nothing. They just pulled a gun and shot him in the chest.”

Butler, who was standing alongside her boyfriend when he was hit, recalled their frantic final moments together as she begged Gurley to keep fighting.

“Yo, you OK? Talk to me!” she recalled shouting. “He wasn’t saying nothing. That was the last thing I said to him.”

Rep. Hakeem Jeffries (D-Brooklyn) angrily dismissed the official explanation of an accidental shooting.

Kimberly Ballinger is also the mother of Gurley's 2-year-old daughter (not pictured).

 (JOE MARINO/NEW YORK DAILY NEWS)

“The senseless killing of another unarmed African-American male by the NYPD should shock the conscience of all New Yorkers and the nation,” said Jeffries. “At this point, talk is cheap.”

Gurley’s sister, along with mother of his daughter, were slated to appear Saturday with the Rev. Al Sharpton.

“I can’t believe my Big Bro is actually gone man,” his sister Akisha Pringle wrote on Facebook. “Just celebrated his 28th birthday a couple days ago. And now he gone.”

Pringle said she received a 3 a.m. Friday call from Butler to break the painful news. Gurley, along with his daughter, was planning to leave on Friday for a Thanksgiving trip to Florida.

“It was going to be a surprise for his mom because she hasn’t seen him in a while,” said Kenneth Palmer, the dead man’s stepfather. “He was going to say, ‘This is my family, mom.’”

The 2-year-old girl asked her mom on Friday afternoon why her father didn’t come to pick her up from day care.

Liang, who joined the force in July 2013, was placed on modified duty after turning in his gun and shield. The Brooklyn district attorney and NYPD internal affairs were both investigating.

As a probationary officer, Liang can be fired without a departmental hearing.

“As we continue to gather the facts, the fatal shooting of this unarmed man is deeply troubling and warrants an immediate, fair and thorough investigation,” said Brooklyn DA Ken Thompson.

Butler said the officers never came down to check on the mortally wounded man, and medical help was only sent after she banged on a neighbor’s door for help.

Police Commissioner Bill Bratton, First Lady Chirlane McCray, and Mayor Bill de Blasio visit the family of Akai Gurley Nov. 21, 2014.

 (JOE MARINO/NEW YORK DAILY NEWS)

“She opened the door and said, ‘Yo, is somebody hurt?’” Butler said, holding a damp washcloth over her red and swollen eyes. “I said, ‘Yeah, my boyfriend.’”

Gurley died at Brookdale University Hospital shortly after his arrival by ambulance, police said.

“They’re going to shoot him in the chest and walk away — like he was a dog!” said Melissa’s mother, 69-year-old Naomi Butler. “He is a human being.”

It took the two officers a full five minutes after the first 911 call to report the shooting, backing the contention of sources that the two rookies were stunned and unsure what to do.

Melissa Butler, 27, is seen here comforted by an unidentified woman. She said that those responsible need to pay for Gurley's funeral.

 (SAM COSTANZA FOR NEW YORK DAILY NEWS)

The darkness in the building stairwell endured despite a request made over the summer to upgrade the lights, sources familiar with the matter told the News. The sources said the request was made in July or August. NYCHA officials declined to comment.

The stairwell remained dimly lit until Friday morning, when a maintenance man was seen fixing a light on the seventh floor. Landings from the fifth through eighth floors were brightly lit by afternoon, while the fourth floor landing remained dark. Residents said this was a common occurrence.

The inexperienced Liang and his equally raw partner were sent to walk the darkened staircases a vertical patrol of the building in the dangerous housing project.

“If they’re rookie cops, I guess they were afraid for their lives,” said Pink Houses resident Veronica Newsome, 66. “I sympathize with them.”

The patrols have previously proven dangerous for cops.

In 1988, housing cop Anthony McLean was gunned down after stumbling on a crack dealer while patrolling a building in Brownsville.

NYPD Officer Brian Groves was shot at close range — and saved by his bullet-resistant vest — while patrolling a lower East Side housing project in Dec. 2012.

“My heart goes out to the family of the young man who was lost,” said Mayor de Blasio. “We don’t know enough but it does appear to have been an accident. And look, we’re going to do a full investigation.”

Local politician Charles Barron condemned the shooting as an outrage.

“They didn’t find a gun,” said the state assemblyman-elect. “And believe me, if he had anything, if he had a slingshot, they would have put that in the report. This is incredible.

“I want to hear the justification for this one.”

Just a short time earlier, Butler had braided her boyfriend’s hair inside her apartment. She said the slain man, who was released from Rikers Island on Oct. 3, was just about to start working for the city.

Gurley had roughly two dozen prior arrests over the past nine years.

Both officers were taken to Jamaica Hospital for treatment of tinnitus.（source: DAILY NEWS）