Two months to the day after a now-former Philadelphia police officer fatally shot 12-year-old Thomas “TJ” Siderio in the back, Edsaul Mendoza turned himself in to authorities and was charged with first-degree murder.

Here’s what to know about the case of the youngest person to have been fatally shot by a Philadelphia police officer in decades, according to the charging documents.

» READ MORE: The ex-Philly Police officer who fatally shot TJ Siderio, 12, has been charged with murder

What happened to TJ?

Around 7:20 p.m. on March 1, TJ and a 17-year-old were riding bikes near 18th and Barbara Streets. Nearby, four plainclothes officers in an unmarked car were staking out the area after seeing a social media post suggesting a teen there had been carrying a gun.

The officers — identified as Mendoza, Kwaku Sarpong, Robert Cucinelli, and Alexander Camacho — were pulling up to the teens because they believed the 17-year-old was “tangentially connected” to a stolen gun case, according to the investigation records. As they activated their emergency lights, identifying themselves as police, the records say TJ shot at the car, shattering the rear window and piercing through a passenger’s headrest. Shards of glass injured Camacho’s eye.

Two officers got out and fired toward TJ, who they said was holding a handgun. The boys ran separate ways, and Mendoza chased after TJ.

» READ MORE: The short, tumultuous life of Thomas ‘TJ’ Siderio

District Attorney Larry Krasner said at a news conference that the shooting was captured on video, and it showed Mendoza shooting at TJ three times — once immediately outside the car, and twice while chasing him up the block.

Krasner said Mendoza fired the third and fatal shot at TJ after the boy had either fallen or dived to the ground, and was no longer armed. Mendoza was about a half-car length away when he shot TJ in the back, while the boy was effectively face-down on the sidewalk, Krasner said.

Krasner said TJ had tossed his Taurus 9MM handgun while running from Mendoza, and the weapon was found about 40 feet from where Mendoza shot him. Video a few minutes later shows Mendoza’s partner asking where the gun was, and Mendoza responding: “He threw it around there,” pointing up the block.

Krasner said this shows Mendoza knew that TJ was unarmed when he chased and shot him.

» READ MORE: Mystery deepens on whether 12-year-old boy was armed when police shot him in the back

Who is Edsaul Mendoza?

All four officers involved had been with the police department for less than six years at the time of the shooting, according to city payroll records.

Mendoza was a five-year veteran assigned to the South Task Force.

A week after TJ was shot and killed, Police Commissioner Danielle Outlaw announced that Mendoza would be fired for violating departmental guidelines.

» READ MORE: The Philly police officer who fatally shot a 12-year-old boy will be fired, Commissioner Danielle Outlaw said

Outlaw previously declined to specify how Mendoza had violated departmental guidelines, except to say: “It was clear that the use-of-force policy was violated.” She said evidence showed it was “certain” that Mendoza — whom she referred to only as “officer number 1″ — fired the shot that killed TJ.

On Sunday night, Mendoza — now a former police officer — surrendered and was charged with counts including first- and third-degree murder, voluntary manslaughter, and possessing an instrument of crime in the death of TJ.

What is the South Task Force?

The South Task Force is a tactical unit that uses social media and on-the-ground surveillance to seize illegal guns and arrest suspects across South Philadelphia. The four officers who approached TJ were part of this force.

The unit plays an important part in one of Outlaw’s prime goals: to get illegal guns off the street.

But people arrested by plainclothes task force officers told The Inquirer that the unit had gained a reputation as “cowboys” or “jump-out boys,” creating unsafe or chaotic situations in pursuit of arrests.

Five people arrested by plainclothes task force officers in the last two years told The Inquirer they initially believed they were being carjacked, robbed, or stalked by some unknown assailant. Others alleged the officers crossed over into misconduct including warrantless searches, physical abuse, and planted guns.

» READ MORE: Before killing a 12-year-old, a South Philly plainclothes police unit caused frequent chaos, residents say

What other repercussions might Mendoza face?

At a press conference, Andrew Duffy, an attorney for TJ’s mother Desirae Frame, said Frame was “devastated” by the details and announced a forthcoming civil lawsuit against Mendoza, the Philadelphia Police Department, and the City of Philadelphia.

“From both the DA Krasner’s press conference this morning as well as the grand jury presentment, it is devastating to learn that this was not just an unjustified shooting. This was murder. We learned that TJ Siderio was executed,” Duffy said.

Duffy said the lawsuit would be filed within a month, but didn’t specify a date.

What do those close to TJ say about the charges?

The details were painful for Frame to learn, said Duffy, but cemented that TJ was shot after heeding police officers’ commands to drop the gun he allegedly had and to drop to the ground. He said no one should fire at a vehicle, but the officers’ plain clothes and unmarked cars may have influenced TJ’s reaction. When asked how TJ may have acquired a gun, Duffy acknowledged the city’s gun violence crisis, but did not say how the 12-year-old might have gotten a weapon.

“There is a horrific gun crisis in Philadelphia and our city needs to do more to rid children and adults of illegal guns. But you do not solve the Philadelphia gun crisis by executing 12-year-olds,” he said.

For Mark Nasuti, a family friend whom TJ considered an uncle, the murder charge was merely confirmation of his view that Mendoza was a rogue officer.

“This guy sounds like he should’ve never been a cop to begin with,” said Nasuti.