The Philadelphia District Attorney’s Office has formally announced charges against a former Philadelphia police officer involved in the fatal shooting of 12-year-old Thomas “TJ” Siderio.

According to CBS Philadelphia, former officer Edsaul Mendoza was changed with first-degree murder, third-degree murder, voluntary manslaughter, and other charges in connection to the March 1 fatal shooting.

During a press conference on Monday (May 2), District Attorney Larry Krasner says the disturbing details of the shooting are contained in a lengthy grand jury presentment.

Siderio was reportedly running from undercover officers at 18th and Barbara Streets in South Philadelphia before Mendoza allegedly shot at the boy three times. The last shot hit Siderio in the back. He died 90 seconds later.

Moments before the shooting, prosecutors say Siderio shot into the rear window of an unmarked police car that was carrying Mendoza and three other plainclothes officers. Police say Siderio dropped his gun as he ran off.

“And police officer Mendoza began what can be fairly called a tactically unsound foot chase of the 12-year-old,” Krasner said, noting that the officer who shot Siderio knew the young boy was unarmed before shooting him in the back.

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Four undercover officers were reportedly parked in an unmarked car in the area of 19th and Barber investigating a teen seen with a firearm on social media. That teen was not Siderio but another one who was with him.

A grand jury found that former officer Mendoza ran onto the sidewalk without showing or reassessing, found Siderio unarmed and not fleeing, and fired a shot into his back from within 10 feet.

“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,” Krasner said. “But he fired a shot through his back, nonetheless, that killed him.”

Siderio first fired a gun into the unmarked vehicle with one officer being struck by glass. That prompted three of the four undercover officers to exit the vehicle and begin tactical maneuvers. Video and audio accounts shown to the grand jury reveal Siderio threw away his weapon some 40 feet from where he drove to the ground and was then shot.

“There’s very strong case law that when you point a firearm and fire it at a vital organ of the body — such as the upper back, the spine, or some other organ — that is sufficient evidence to establish the elements of first-degree murder,” Krasner said.

CBS Philadelphia reports that the medical examiner’s office determined the manner of death as homicide. Mendoza surrendered early Monday morning and is currently being held without bail.