A dramatic day in court as a jury begins hearing the case against NYPD Officer Peter Liang. He's on trial for manslaughter in the shooting death of an unarmed man in the stairwell of a public housing development in East New York, Brooklyn.

"This man, Peter Liang, with a flashlight in one hand, and his gun in the other, bust into the stairwell on the eighth floor, turned left, pulled the gun's trigger," said Assistant District Attorney Marc Fliedner in his opening statement.

A dramatic opening in the manslaughter trial of officer Peter Liang.

Fliedner told the jury Liang recklessly fired his gun in an unlit stairwell, argued for minutes with his partner about calling their supervisor, and then did nothing to help Akai Gurley once he realized the bullet hit Gurley in the chest one flight below. 

"This man had just shot an innocent man, and he never even knelt to try and undo what he had done. To look into his dying eyes. To, even touch him."

Liang stared straight ahead during the prosecutor's arguments.

His defense attorney said Liang was a young cop who did nothing wrong when his gun accidentally fired.

"He has his gun out because he's on his way to the roof, the most dangerous place, in a dangerous place," said defense attorney Rae Koshetz.

She said the evidence will show that this was a "million to one possibility" – the bullet ricocheting off the stairway wall, killing Gurley.

A Pink Houses resident, Melissa Lopez, testified for the prosecution. She called 9-1-1 to help Akai Gurley's girlfriend try and save his life. That tense call was played in court.

Dispatcher: "Ma'am, you said the cops are there?"

Voice: "He's not breathing!"

Melissa Lopez: "He’s not breathing!"

Dispatcher: "All right, his girlfriend, is she willing to do CPR?"

Melissa Lopez: "Is she willing to do CPR, mama?

Dispatcher: "Tell her to breathe in his mouth."

Melissa Lopez: "She said breathe in his mouth."

Lopez said Liang and his partner did nothing to help.

In terms of the jurors, they are mostly white. There's one black man.

The judge told all of them, they were selected because they said they could be fair and impartial.

The defense lawyer emphasized that point, telling jurors the case was "not a referendum on policing in the U.S."

Officer Liang's partner is expected to testify for the prosecution. Liang may testify in his own defense.