New York City Police officer Peter Liang arrives at Brooklyn Supreme Court for a pre-trial hearing, for the fatal shooting of an unarmed man while patrolling the stairwell of a Brooklyn housing project, in New York, May 14, 2015. REUTERS/Stephanie Keith

As Akai Gurley bled to death in a New York stairwell, the police officer who accidentally shot him was upstairs arguing with his partner about whether to call in the incident, prosecutors said at the start of the officer's trial Monday.

New York Police Department Officer Peter Liang is charged with manslaughter for shooting Gurley in a Brooklyn public housing project on Nov. 20, 2014.

"Akai Gurley is dead today because he crossed paths with Peter Liang," Brooklyn Assistant District Attorney Marc Fliedner told jurors in an impassioned opening statement. He said Liang was reluctant to phone in the in incident because he was afraid he would be fired.

But Rae Koshetz, Liang's defense lawyer, said the shooting was an accident and that Liang had no idea the stray bullet had ricocheted off a wall and struck Gurley's chest in the "pitch black" stairwell.

"The evidence in this case will show that this was a million-to-one possibility," she said.

The death of the 28-year-old Gurley, an unarmed black man, added to nationwide tensions over police use of force against minorities.

Just days after Gurley's death, a grand jury declined to indict a white police officer for killing black teenager Michael Brown in Ferguson, Missouri. Less than two weeks later, a New York grand jury cleared a white officer in the chokehold death of Eric Garner.

Koshetz addressed that issue head on, telling jurors that the case was "not a referendum on policing in the United States." The 12-member jury appeared overwhelmingly white.

Both sides agree that Liang drew his weapon as he and his partner, Shaun Landau, entered the unlit stairwell on patrol. Meanwhile, Gurley and his girlfriend were walking one floor below on their way out of the building.

Prosecutors have not accused Liang of intending to shoot anyone. But they said he acted recklessly in unholstering and firing his weapon and accused him of failing to check whether the bullet hit anyone because he was too worried about losing his job.

Liang also did not administer first aid to Gurley after realizing he had been hit, prosecutors said.

But Koshetz said Liang was in a "state of shock" following the shooting, eventually requiring his own ambulance, and was in no condition to help anyone.

Liang's lawyers have said he will likely take the stand to tell jurors what happened. His partner is expected to testify for the prosecution under an immunity agreement.

The trial is expected to last several weeks.