(AP) -- A friend of an unarmed man who was shot by a rookie New York Police Department officer in a darkened housing project stairwell can be heard wailing in the background on a recording of a 911 call that was played publicly for the first time Monday at the officer's manslaughter trial.

"He's not breathing!" the friend, Melissa Butler, yelled as the caller stood nearby and relayed a medic's phone instructions for CPR on the night of Nov. 20, 2014.

The recording also captured a brief exchange between the caller, a resident who heard the shot, and defendant Peter Liang and his partner. The uniformed officers appeared while Butler -- her hands covered in blood -- was frantically trying to save the life of Akai Gurley. But they never offered any assistance as they descended the stairwell, Melissa Lopez told a Brooklyn jury.

Liang "didn't do nothing," Lopez said.

Prosecutors sought to use the tape and Lopez's testimony on the first day of Liang's trial to show that, along with recklessly firing his weapon, he callously ignored his duty to aid his victim.

"A police officer -- this police officer -- shot an innocent person . and he never even knelt down and try to fix what he'd done," prosecutor Marc Fliedner said in opening statements Monday.

The defense claims Liang, because his weapon accidentally discharged, didn't commit a crime. The officer has pleaded not guilty to second-degree manslaughter, criminally negligent homicide and other charges.

"Peter Liang had no intent to hurt anybody," said defense lawyer Rae Koshetz.

The Liang trial is being closely watched by advocates for police accountability, who see it as a counterpoint to decisions by grand juries declining to indict white police officers in other killings, including those of Eric Garner on Staten Island and Michael Brown in the St. Louis suburb of Ferguson. Meanwhile, Chinese-American supporters of Liang say he has been made a scapegoat for past injustices.

At the time of the shooting, Liang, an officer for 18 months, and his partner were patrolling the Brooklyn housing project amid reports of a spike in violent crime. The officer had his gun drawn as he entered the stairwell on the eighth floor because he was headed to the roof -- "the most dangerous place of a dangerous place," his lawyer said.

The 28-year-old Gurley and Butler had already entered the door into the seventh-floor landing to head down to the exit. Liang -- his gun in his left hand and using a flashlight in his right because the lights were burned out -- fired a shot that ricocheted and hit Gurley, who made it down two flights of stairs before collapsing.

Koshetz said Liang initially had no idea the bullet had struck anyone. Once he learned, "he was in a state of shock and was hyperventilating," she said.

But prosecutors say after the gun went off, rather than check to see if someone was hurt, Liang repeatedly told his partner it was an accident, argued over which one should report it and fretted about what it would mean for him.

Liang "stood there whining and moaning about how he could get fired," Fliedner said.

Prosecutors are expected to call Butler and Liang's partner as their key witness. Liang also is expected to testify in his own defense.

Gurley's slaying recalled two others by officers patrolling Brooklyn housing projects -- the shootings of 19-year-old Timothy Stansbury on a rooftop in 2004 and of 13-year-old Nicholas Heyward Jr. while carrying a toy gun. Neither officer was charged.

Gurley's family has brought a wrongful-death lawsuit on behalf of his estate and his young daughter.