On Monday, May 2, the Philadelphia District Attorney’s Office announced charges against former police officer Edsaul Mendoza, who was involved in the fatal shooting of 12-year-old Thomas “TJ” Siderio.

Former officer Mendoza was charged with first-degree murder, third-degree murder, voluntary manslaughter, and other charges in connection to the March 1 shooting.

Police say that Mendoza shot and killed Siderio in South Philadelphia, shooting him in the back as he ran away. Moments before, police say Siderio shot into the rear window of an unmarked police car with Mendoza and three other plainclothes officers in it. He then dropped the gun and ran off.

On Monday, May 2, new details of the shooting were revealed in unsealed grand jury documents. Siderio had thrown a gun down about 40 feet before he was shot, and he dropped to the ground, either tripping or obeying a command to get down.

In a press conference, Philadelphia District Attorney Larry Krasner said the officer crossed between two parked cars, and from about a half-car length away, fired the fatal shot. Krasner also said that Mendoza knew the young boy was unarmed before shooting him in the back.

Krasner said that much of the evidence was based on a video that has not yet been released to the public.

“It is certain that [Siderio] had stopped running and he was possibly surrendering, and he was essentially facedown on the sidewalk,” Krasner said, saying that the young boy was in a pushup position looking at Mendoza.

Krasner also said that the entire foot-chase was “tactically unsound” and that the video was disturbing to watch.

The new details reveal that four undercover officers were parked in an unmarked car on 18th and Barber Streets investigating a teenager seen with a firearm on social media. This teen was not Siderio, but another young boy who was with him at the time.

The grand jury finds 12-year-old Siderio first fired a gun into that unmarked unit. One officer was struck by glass. This prompted three of the four officers to leave the car and begin tactical maneuvers that have been criticized as a departmental policy violation.

The jury concluded that officer Mendoza, who has since been fired from the force, ran onto the sidewalk — without slowing or reassessing — finding Siderio unarmed and not fleeing, and fired that shot into his back from within 10 feet.

“The gun was sitting on the street, below the curb line, nearly 40 feet away. 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. 

Court records show Mendoza surrendered on Sunday, May 1 and was denied bail, which is rare treatment for former law enforcement officers facing charges. 

A spokesperson for the Fraternal Order of Police Lodge 5 said the union plans to provide Mendoza with an attorney. 

Philadelphia Police Commissioner Danielle Outlaw fired Mendoza after the shooting, saying he violated the department’s use of force directive. 

CBS Eyewitness News spoke with Siderio’s great-grandmother about the emotion of Monday’s news. 

“I’m happy. That’s all I can say right now. I’m so heartbroken, I can’t sleep. None of us can sleep. It’s horrible,”  Siderio’s great-grandmother, Mary Siderio, said.