A rookie police officer was arguing with his partner over who should call their supervisor in the chaotic moments after he had accidentally fired his gun into a darkened stairwell of a public housing complex, and he did nothing to help when he discovered later that he had mortally wounded an unarmed man, the prosecution says in court papers.

Officer Peter Liang is charged with manslaughter over the death of Akai Gurley on 20 November in the Louis Pink Houses in Brooklyn. He has pleaded not guilty and his lawyer notes the shooting was an accident.

Prosecution motions released on Tuesday gave the following details.

Liang and his partner, Shaun Landau, were on patrol on the eighth floor of one of the buildings at about 11pm when they walked into the stairwell.

Liang held his flashlight over his head and had his Glock pistol pointed directly in front of him when he started to walk down the stairs. His partner was still in the hallway when he heard a gunshot. At the same time, Gurley and his girlfriend, Melissa Butler, were on the landing of the staircase below. The bullet had bounced off a wall before striking Gurley. The couple managed to get down several flights of stairs before Gurley collapsed.

Liang ran out of the staircase and his partner, using an expletive, asked what happened.

“It went off by accident,” Liang said, then repeatedly exclaimed he would be fired.

The two stood in the hallway and argued for several minutes about who should call their supervisor to report the gunshot and what phone should be used.

“You call,” Liang told his partner.

“No, you call,” Landau said.

But no one called. Instead, Landau went into the stairwell, searching the walls for bullet holes, but soon heard a “grunting noise” coming from the floors below. When he reached the fifth floor, he saw Gurley’s body and Butler kneeling over him, tears pouring down her face.

By then, Butler was on the phone with a 911 operator, who was trying to walk her through performing CPR as the officers stood nearby.

“Neither defendant nor Officer Landau provided any medical care to Mr Gurley. Nor did they summon an ambulance,” prosecutors wrote in the court filing. Instead, the two of them walked around Gurley’s body to the landing on the fourth floor.

It was nearly 20 minutes after the shooting when the officers radioed to report “an accidental fire”.

Liang was later indicted on charges including manslaughter, criminally negligent homicide, official misconduct and assault. Prosecutors said he disregarded his training and should not have had his gun drawn nor his finger on the trigger.

Liang pleaded not guilty in February and is currently free without bail.

His attorney Stephen Worth, who had tried to get the court to dismiss the indictment, said on Tuesday, “We wish the judge would have taken a more intensive look at the grand jury presentation. We believe it was not a fair presentation for officer Liang.”

The case was closely watched following the grand jury decision on 3 December not to indict a white police officer over the chokehold death of Eric Garner, an unarmed black man who was arrested on Staten Island. That decision – along with a grand jury’s refusal to charge a white officer in the shooting of Michael Brown, an unarmed black 18-year-old, in Ferguson, Missouri – prompted mass protests decrying the grand jury system as biased. Liang, 27, is Asian; Gurley, 28, was black.

On Tuesday, New York took a step to give such cases special consideration by appointing the attorney general to investigate them, at least for a year. The Brooklyn district attorney, Kenneth Thompson, who presented evidence to the grand jury against Liang, has opposed the idea.

Brooklyn supreme court judge Danny Chun ruled that evidence presented to the grand jury was legally sufficient to support manslaughter charges against the officer. Liang is due back in court in September.