PHILADELPHIA (CBS) – The Philadelphia District Attorney’s Office announced charges against a former Philadelphia police officer involved in the fatal shooting of 12-year-old Thomas “TJ” Siderio. Former officer Edsaul Mendoza was charged with first-degree murder, third-degree murder, voluntary manslaughter, and other charges in connection to the fatal shooting on March 1.

The disturbing details, as District Attorney Larry Krasner called them at a news conference Monday morning, are contained in a lengthy grand jury presentment.

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It’s alleged Siderio was running from undercover officers at 18th and Barbara Streets in South Philadelphia.

Mendoza is accused of chasing and shooting at the boy three times. The last shot hit Siderio in the back. It’s said he died in 90 seconds.

Moments before, 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.

Krasner said Monday during a press briefing the officer who shot Siderio knew the young boy was unarmed before shooting him in the back.

The investigation shows four undercover officers were parked in an unmarked car in the area of 18th and Barber investigating a teen seen with a firearm on social media. That teen was not Siderio but another young man who was with him.

A grand jury found 12-year-old Siderio first fired a gun into that unmarked unit. One officer was struck by glass. That prompted three of the four undercover police officers to exit the vehicle and begin tactical maneuvers that have been criticized as departmental policy violations. It concludes 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.

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“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.

Video and audio accounts shown to the grand jury reveal Siderio threw away his weapon some 40 feet from where he dove 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.

The medical examiner’s office found that the manner of death is homicide.

Mendoza surrendered early Monday morning. He is currently being held without bail.

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

The Fraternal Order of Police released a statement, saying: “The Philadelphia Fraternal Order of Police will represent this officer against these very serious charges. The accused officer, like every other citizen, is entitled to due process and we are confident that our judicial system will protect this officer’s constitutional right to a fair trial.”

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

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

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CBS3’s Joe Holden contributed to this report.