Prosecutors have said the boy was on the ground and unarmed when Edsaul Mendoza fired the fatal shot.
Mendoza also pleaded guilty to possession of an instrument of crime as part of a plea deal with the Philadelphia District Attorney's Office.
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Mendoza had been charged with first- and third-degree murder and voluntary manslaughter in the shooting of Thomas "T.J." Siderio in March 2022, with the district attorney's office saying at the time that video contradicted the officer's version of events.
The Siderio family said Friday they are upset with the plea deal and wanted the original charge of first-degree murder to stick.
READ MORE: Former Philadelphia police officer charged with murder in shooting of 12-year-old boy
"It's heartbreaking, it's earth-shattering, it's devastating. They got it wrong. He should be held to a higher standard than you and I," said the boy's aunt, Joanne Payne. "We cannot stand over a child on a city street and shoot them. TJ was on the ground, face down, unarmed. The threat was eliminated."
Prosecutors acknowledged that not all family members were happy with the plea deal but said they did take their input into consideration.
"They painted a picture of a horrendous crime, and the sentencing that they are offering is nowhere near the balance of the scale," added Payne.
Authorities say Mendoza chased the child after prosecutors claim the boy fired a shot at the unmarked police car and ran -- something family members dispute.
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Siderio threw a gun down about 40 feet before he was shot and then dropped to the ground, either tripping or obeying a command to get down, according to authorities.
The four officers had been in an unmarked car, looking for a teenager they wanted to interview related to a firearm investigation, police have said. They saw Siderio and an unnamed 17-year-old, and maneuvered the car around the block and next to them to initiate a stop.
Prosecutors said that almost at the same time the officers turned their red and blue lights on, a shot came through the back passenger window and ricocheted around the car. One officer was treated for injuries to his eye and face caused by broken glass.
Mendoza and another officer on the passenger side got out and fired one shot each, according to police. Mendoza then chased Siderio down the block, firing twice and striking the boy once in the back from what prosecutors say was "relatively close range."
Mendoza, a five-year veteran of the force, was fired a week after the shooting by then-Police Commissioner Danielle Outlaw, who said his conduct violated department policy.
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READ MORE: Family of 12-year-old fatally shot by Philadelphia police in 2022 files lawsuit against city
Siderio's family sued Mendoza and the city in January, saying his death was the result of "an abysmal systemic policy failure" within the police department.
Mendoza is scheduled to be sentenced in July.
The Associated Press contributed to this report.