A Philadelphia police officer who was fired for fatally shooting a 12-year-old unarmed boy in the back has been charged with murder.

A grand jury charged Edsaul Mendoza with first-degree murder, third-degree murder, voluntary manslaughter and possessing an instrument of crime in the March 1 killing of Thomas “TJ” Siderio, Philadelphia District Attorney Larry Krasner said at a news conference Monday.

Mendoza surrendered on Sunday night, according to court records, and was jailed without bail.

Four plainclothes officers, including Mendoza, were in an unmarked car in South Philadelphia when they pulled up to TJ and a 17-year-old. The district attorney’s office said the four officers gave inconsistent explanations for the stop, though Krasner said one reason given was because the youths were riding their bicycles the wrong way.

TJ “likely” fired a shot at the police car about the same time officers activated the police lights, the district attorney said. Mendoza got out of the car and began a “tactically unsound” foot chase for TJ, firing a total of three shots, Krasner said. The third bullet hit TJ in the back and exited through his chest.

Krasner said the shooting was captured on video viewed by grand jurors that shows Mendoza firing the last shot after the boy was “effectively face-down” after either diving or falling to the ground. Mendoza was “half a car length away” — just a few feet — when he fired the fatal shot, the DA said.

“I find this very disturbing and very difficult to watch,” Krasner said of the video, which still hasn’t been released to the public.

A gun the prosecutor said TJ used to fire at the police car was found about 40 feet from his body, which means he was unarmed when Mendoza shot him. TJ disposed of the firearm before Mendoza fired the last two shots, according to the district attorney.

Mendoza appeared to know that TJ was unarmed before he shot the boy, Krasner said. The video shows the officer running toward TJ without fear of being shot, Krasner said, and also shows Mendoza indicating to a fellow officer where the boy tossed the weapon.

Police Commissioner Danielle Outlaw fired Mendoza about a week after the killing, saying his conduct violated department policy.

The department has refused to confirm the identities of the other officers involved, citing credible threats to them. Krasner said he has no information about such threats.