Charles Barron speaks during a news conference with Melissa Butler...

Charles Barron speaks during a news conference with Melissa Butler at the Office of the Brooklyn District Attorney in Brooklyn on Monday Nov. 24, 2014. Butler was with Akai Gurley when he was shot inside the stairwell. Credit: Theodore Parisienne

The city medical examiner's office said Sunday that Akai Gurley was killed last week by a single gunshot to the torso, which officials said was fired by a rookie police officer, and ruled his death was a "homicide."

A finding of homicide means that Gurley's death was caused by the action of another person and is not a finding of criminal culpability or legal fault, a spokeswoman for the medical examiner said.

The autopsy result wasn't a surprise and comes as some politicians were pressing the Brooklyn district attorney's office to move forward with a criminal investigation into the actions of Officer Peter Liang.

Experts in police tactics said the key to whether there is a criminal case will be whether Liang was negligent in not only unholstering his Glock 9-mm pistol but also in moving his finger from the trigger guard to the trigger. A single shot from Liang's service weapon the night of Nov. 20 struck Gurley, 28, as he walked up a darkened stairwell with his girlfriend in the high-crime Pink Houses public-housing in East New York, Brooklyn. Police Commissioner William Bratton said the shooting was accidental.

Liang, 25, was on a vertical patrol with a rookie partner and had his gun in his left hand as he opened the stairwell door. Liang held a flashlight in his right hand but it is still unclear which hand he used to open the door. Immediately after the incident, Liang said the shooting was accidental, a law-enforcement official said.

Monday, Assemb.-elect Charles Barron (D-Brooklyn) and others including Gurley's girlfriend Melissa Butler met with Brooklyn District Attorney Kenneth Thompson, a spokeswoman for Thompson said. However, the prosecutor had no comment about the meeting. Barron has been pressing the case for criminal charges against Liang.

But the law-enforcement official familiar with the investigation said a decision by Thompson about whether Liang should be charged isn't expected for a month. Liang, a probationary officer on the force less than two years, is on modified duty with no shield or gun.

The shooting was something in which "everything went wrong," said former NYPD Det. Sgt. Joseph Giacalone, who, as a rookie in 1994, got into a gunfight with five suspects in which no one was injured.

"Having a gun out is not an issue, having a finger on the trigger is what they are going to focus on," Giacalone, who teaches investigative techniques to police departments, said. "That is what the investigation is going to have to reveal -- what transpired in those microseconds to have that officer fire his gun."

One police source said the investigation will have to determine if Liang was hit by the self-closing stairwell door.

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Totally tubular trips ... LI Works: Talking turkey ... Get the latest news and more great videos at NewsdayTV

Get the latest news and more great videos at NewsdayTV Credit: Newsday

Totally tubular trips ... LI Works: Talking turkey ... Get the latest news and more great videos at NewsdayTV

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