A former Philadelphia police officer was charged with murder Monday in the death of a 12-year-old boy who a prosecutor alleged was shot in the back after apparently complying with orders to toss down a gun and get on the ground.

Philadelphia District Attorney Larry Krasner said during a news conference that former undercover cop Edsaul Mendoza was indicted by a grand jury on charges of first-degree murder, third-degree murder, voluntary manslaughter and possession of an instrument of a crime.

The charges stem from the fatal shooting of Thomas “T.J.” Siderio Jr. in South Philadelphia on March 1.

Siderio was shot after Mendoza and three other undercover officers in an unmarked car attempted to stop him and a 17-year-old boy who were riding their bicycles in the wrong direction on a street, authorities said. Krasner said Siderio was initially armed with a handgun and fired a shot through a rear window of the unmarked police car, setting off the fatal foot chase.

Krasner said video of the shooting that was played for the grand jury appeared to show Mendoza ordering Siderio to put down the gun and get on the ground just seconds before he was shot.

The district attorney described the foot chase by Mendoza as being “tactically unsound” and that the former officer fired three times at Siderio, two of which missed.

At the time of the last two shots fired by Mendoza, Siderio was unarmed, having discarded a 9mm gun 40 feet from where he was shot, Krasner said.

“It is certain that Thomas Siderio, at the time he was shot, had stopped running and was possibly surrendering,” Krasner said. “It is certain that Thomas Siderio, at the time he was shot, was essentially face down on the sidewalk, that he was in a position that approximates sort of a push-up … perhaps turning to look at the officer who was pursuing him when he was shot through the back.”

The prosecutor alleged that Mendoza knew Siderio was unarmed and not a threat when he shot the boy.

Krasner said Mendoza was standing close to Siderio when he shot the boy.

He said that prior to firing the fatal shot, Mendoza allegedly approached Siderio in a manner that was “completely inconsistent with Mendoza believing that Siderio was armed.”

“He (Mendoza) was within half a car length of Thomas Siderio and thus would have had the opportunity to see Thomas Siderio clearly at the time he fired,” Krasner said.

Krasner said immediately after shooting Siderio, Mendoza told another officer where Siderio “threw the gun,” and even pointed to the location of the weapon, found on the street at the edge of the sidewalk.

“Thus, when officer Mendoza fired the third and fatal shot, he knew the 12-year-old, 5-foot tall, 111-pound Thomas Siderio no longer had a gun and no ability to harm him, but he fired a shot through his back nonetheless that killed him,” Krasner said.

Krasner also said it remains unclear if Siderio realized that Mendoza and the other officers who pulled up to him and his friend in an unmarked car were police officers. He said the officers were all wearing street clothes and vests with no marking on the front indicating they were police.

“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,” Krasner said. “It is very concerning.”

A bail hearing for Mendoza was held Monday morning, and a judge ordered him to be jailed without bail.

It was not immediately clear if Mendoza has hired an attorney.

About a week after the shooting, Philadelphia Police Commissioner Danielle Outlaw announced she was suspending Mendoza, a five-year veteran of the police force, for 30 days, after which she intended to dismiss him for violating the department’s “use-of-force directive.” Mendoza has since been fired.

“It’s tragic that we have trigger-pullers as young as 12,” Outlaw said at the time. “And it’s tragic that we had one of our own, again, go against everything who we say we are. There are no winners here.”