A former Philadelphia police officer is charged with murder in a 12-year-old's death

Former Philadelphia police officer Edsaul Mendoza knew Thomas "T.J." Siderio, 12, was unarmed when he shot him in the back and killed him, Philadelphia District Attorney Larry Krasner said as he announced murder charges against Mendoza on Monday.

Police said Siderio had fired at a car carrying four officers before running away on March 1 — but Krasner said Mendoza, who chased Siderio on foot, knew the boy had thrown his weapon away before he shot him. Siderio was on the ground at the time, Krasner said.

"It is certain that Thomas Siderio, at the time he was shot, had stopped running and that he was possibly surrendering," Krasner said.

"It is certain that Thomas Siderio, at the time he was shot, was essentially facedown on the sidewalk, that he was in a position that approximates sort of a pushup. Turning back toward where the officer was pursuing him, perhaps turning to look at the officer who was pursuing him, when he was shot through the back."

A Taurus 9 mm pistol was recovered about 40 feet from Siderio's body, Krasner said.

Mendoza, who was fired soon after the incident, is now in custody and is being held without bail, Krasner said as he announced the murder charges. In addition to first-degree murder, he is charged with third-degree murder, voluntary manslaughter and possession of an instrument of crime.

Mendoza also made an "untruthful statement about his location" when he was chasing and firing at Siderio, the prosecutor said, adding that it suggests the former officer knew what he had done was illegal.

New details emerge about the incident 2 months ago

Citing the findings of a grand jury, unreleased video of the encounter and interviews with those present, the district attorney's office released the most detailed view yet of what investigators believe happened.

Mendoza and three other officers were wearing plainclothes and riding in an unmarked police car when they drove up on two juveniles riding bikes.

As the car approached, the vehicle's police lights weren't on. Krasner said the timing of the lights being activated and the first shot ringing out was nearly simultaneous.

"When the child fired the gun, that immediately caused three officers to take cover, and police officer Mendoza began what can be fairly called a tactically unsound foot chase of the 12-year-old," Krasner said.

Mendoza fired his weapon three times: once as he began chasing Siderio down the block, again in the middle of the block and finally at the end of the block, "while standing on the sidewalk and relatively close to Thomas Siderio," Krasner said.

Mendoza fired his second shot as he ran past the gun on the ground. The bullet didn't hit Siderio, but the 12-year-old went to the ground almost simultaneously, Krasner said, either because he fell or dove to the ground. He was on the ground for about four to six seconds before being fatally shot, prosecutors said.

Immediately after the shooting, Mendoza told another officer that Siderio had thrown the gun away and pointed to the spot where it was found next to a curb, Krasner said.

"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.

The officers involved in the incident gave differing reasons for the initial decision to stop Siderio and the other minor. Two of the officers said they wanted to speak to the pair as part of a weapons inquiry. Two others said they were making a traffic stop related to riding a bike the wrong way on the street.

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