The NYPD officer who shot and killed an unarmed Brooklyn man in 2014 testified at his trial Monday and became emotional as lawyers pressed him about the deadly chain of events that took place. NY1's Dean Meminger filed the following report.

Officer Peter Liang broke down in tears on the witness stand as he described to his lawyers seeing the body of Akai Gurley, the man he says he accidentally shot to death.

But what was it that made the rookie officer fire his gun when he entered a dark staircase inside the Pink Houses in Brooklyn?

"Soon as I got in, I heard something," Liang said in testimony Monday. "The gun just went off when I tensed up."

Suggesting he was startled by what he described as a sudden sound.

Gurley's parents say they weren't moved by the officer's crying while testifying in his own defense.

"It's not an accident, OK? He murdered my son, and I want justice for my son," said Sylvia Palmer, Akai Gurley's mother.

"To be honest, if he had apologized a long time ago, I would have felt it," said Kenneth Palmer, Gurley's stepfather.

Liang says after noticing the stairwell was dark, he put his flashlight in one hand and his gun in other. He said he initially tried to open the door with the hand holding the flashlight, but couldn't. So he used his shoulder instead.

The defense asked if he had his finger on the gun's trigger. He replied, "It was on the side of the weapon, along the frame." He went on to say, "There's no need to put my finger on the trigger unless I intend to shoot."

The officer said he never had a problem while doing hundreds of vertical patrols in public housing.

Several parents of people killed by officers stood with Gurley's family to say they want Liang sent to prison.

"This was not no tragic accident and it wasn't a mistake. And the gun did not go off by itself," said Nicholas Heyward Sr., whose son was killed by police.

Liang says he did not initially use his radio to call for help because he did not want a bunch of officers responding and leaving their posts to look for the bullet.

The Brooklyn DA says Liang was reckless in handling his gun and in not providing CPR to Gurley after realizing he shot the man.

During closing arguments, Brooklyn prosecutors will try to hammer home their point that Liang's actions were criminal. The officer's defense team will say the shooting was only a tragic accident.