The former Philadelphia Police officer who shot a 12-year-old boy in the back in March has been charged with murder after prosecutors said he fired the fatal shot from near-point blank range while the boy was on the ground and unarmed.

Edsaul Mendoza, 26, surrendered Sunday night and faces counts including first- and third-degree murder, voluntary manslaughter, and possessing an instrument of crime in the death of Thomas “TJ” Siderio, court records show. Mendoza was jailed and denied bail.

District Attorney Larry Krasner said at a news conference that the shooting was captured on video, and that it showed Mendoza firing the last of three shots at TJ after the boy either fell or dove to the ground while running from police along a South Philadelphia street. Mendoza was about half a car length away when he fired into TJ’s back while the boy was effectively facedown on the sidewalk, the DA said.

The shooting came moments after TJ fired a shot at an unmarked car carrying Mendoza and three other plainclothes officers, Krasner said. But he said that TJ had tossed his gun while running away, and that the weapon was found about 40 feet from where Mendoza shot him.

Krasner said evidence showed that Mendoza knew that TJ — 5 feet tall and 111 pounds — was unarmed when he shot the boy, and that his conduct before and after the shooting made that clear.

“You have an unarmed 12-year-old who drops a gun 40 feet from where he was shot to death. At the time he was shot, he was on the ground, not trying to get away,” Krasner said. “I find this very, very disturbing, and very difficult to watch.”

Police Commissioner Danielle Outlaw fired Mendoza, a five-year veteran, a week after the incident, saying his conduct violated department policy. Department officials declined to comment Monday, citing ongoing criminal and administrative investigations.

Mendoza’s lawyers also declined to comment. John McNesby, president of the Fraternal Order of Police Lodge 5, said in a statement that Mendoza, “like every other citizen, is entitled to due process and we are confident that our judicial system will protect this officer’s constitutional right to a fair trial.”

Andrew Duffy, an attorney for TJ’s mother, Desirae Frame, said she was “devastated” by the details of Krasner’s briefing.

“This was not just an unjustified shooting. This was murder,” Duffy said. “We learned that TJ Siderio was executed.”

Mark Nasuti, a family friend who considered TJ a nephew, said the murder charge affirmed his belief that Mendoza did not belong on the force.

”This guy sounds like he should’ve never been a cop to begin with,” Nasuti said.

Mendoza is accused of shooting TJ near 18th and Barbara Streets in South Philadelphia around 7:30 p.m. on March 1.

In a grand jury presentment unsealed Monday, prosecutors said he and three other plainclothes officers — Kwaku Sarpong, Robert Cucinelli, and Alexander Camacho — had set out that day to search for a 20-year-old suspect in a stolen-gun investigation. They were all assigned to the South Task Force, a group that frequently encountered, if not caused, chaos. The four officers had crammed into a single Chevrolet Cruze with tinted windows due to a shortage of other available vehicles, the presentment said.

>> READ MORE: Before killing a 12-year-old, a South Philly plainclothes police unit caused frequent chaos, residents say

For about an hour before the shooting, the grand jury said, the four Task Force officers sat in their car on 18th Street, where they believed the suspect might be, before seeing two young people on bicycles: TJ and a 17-year-old, who is not named in charging documents. Camacho believed the older teen was a friend of the gun suspect, the panel said.

The officers decided to stop the boys even though prosecutors noted police directives generally say uniformed officers should conduct such stops — not those in plainclothes. The presentment said officers testifying before the grand jury gave different justifications for the stop: Sarpong and his supervisor, Sgt. Vincent Butler, said the officers wanted to talk to the 17-year-old, while Camacho and Cucinelli said they wanted to issue the boys tickets for riding their bikes the wrong way on the street.

Before they could initiate the stop, however, Sarpong flicked on the car’s police lights, the grand jury said — and almost simultaneously, a bullet pierced the vehicle’s back window. Krasner said evidence showed TJ had likely fired the shot.

Still, the DA questioned whether the entire episode could have been avoided with the presence of a marked car or uniformed officers. “This is the kind of encounter that could cause someone on the street to believe that the people who are pulling up are not police at all — to believe that the people who are pulling up, in a climate that is obviously rife with gun violence, are pulling up to do them harm and are not law enforcement at all,” he said.

Camacho was injured when shattered glass entered his eyes, the grand jury said. Mendoza and Sarpong then got out and each fired back at TJ. Sarpong did so after taking cover behind a parked car, the panel said, while Mendoza ran after the boy.

Surveillance video captured TJ running up the street, the grand jury said, followed by Mendoza a few seconds later. After dropping the gun, the panel said, TJ dove or fell to the ground, while Mendoza continued running after him.

Assistant District Attorney Brian Collins, speaking at the morning news conference, estimated that TJ was on the ground for four to six seconds before Mendoza approached and shot him in the back.

Krasner said Mendoza appeared to know at that point that the boy was unarmed — that he ran toward him without fear, cover, or backup, and that in the seconds after the shooting, Mendoza was captured on video telling a fellow officer to look for gun farther down the block, where it was later found.

“Mendoza knew that this child did not have a gun in his hand,” Krasner said.

>> READ MORE: 12-year-old TJ Siderio was shot five doors down from where he’d tossed the gun, recordings show

“When people make untruthful statements about what happened that are crucial to understanding the death of another person, in the law, that can be interpreted to indicate guilty knowledge — a sense that you need to cover something up because you know that what you did is illegal,” Krasner said.

The shot that struck TJ in the back, meanwhile, killed him within 90 seconds, the grand jury said.

Mendoza is the third Philadelphia officer to be charged with murder in an on-duty shooting since Krasner took office in 2018. The other two — Ryan Pownall and Eric Ruch — are awaiting trial on third-degree murder and related counts.

In Pownall’s case, Krasner’s office has asked the Pennsylvania Supreme Court to weigh in on an issue that could take on relevance in Mendoza’s case: Prosecutors have asked the court to strike down a part of state law that allows officers to shoot to prevent the escape of a fleeing felon, saying it is unconstitutional.

The court has not given an indication when it may issue a ruling.

Staff writer Samantha Melamed contributed to this article.