The Philadelphia police officer who fatally shot a 12-year-old boy in the back last week will be fired, Commissioner Danielle Outlaw said Tuesday.

Outlaw said the officer will be suspended for 30 days with intent to dismiss, the process by which officers are typically removed from the force.

She declined to identify the officer, citing potential threats to his safety. But police sources with direct knowledge of the investigation said the officer was Edsaul Mendoza, a five-year veteran assigned to a task force in South Philadelphia. Attempts to reach him for comment Tuesday were unsuccessful, and the police officers’ union representing him declined to comment.

Outlaw 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″ — was the one who fired the shot that struck and killed Thomas “TJ” Siderio.

And while she declined to provide many details on what she called an ongoing investigation, she said the shooting — which she said happened after the boy fired a shot into an unmarked police car and took off running — was “a sickening and saddening situation all around.”

“It’s tragic that a 12-year-old had access to a gun. It’s tragic that we have trigger pullers as young as 12,” Outlaw said. “And it’s tragic that we had one of our own go against everything that we say we are.”

Mayor Jim Kenney said in a statement that he supported Outlaw’s decision, calling the shooting a “deeply troubling incident with a tragic loss of life.”

District Attorney Larry Krasner also called the case “deeply troubling,” but declined to comment further as his office investigates and weighs potential criminal charges.

The update came a week after TJ was fatally shot by officers near 18th and Barbara Streets in South Philadelphia. Police have said four plainclothes officers assigned to the South Task Force were in an unmarked car around 7:24 p.m. on March 1, staking out the area after seeing a social media post suggesting a teen there had been carrying a gun.

The officers were Mendoza, Kwaku Sarpong, Robert Cucinelli, and Alexander Camacho, according to police records obtained by The Inquirer. Outlaw on Tuesday declined to confirm their identities, citing what she said were threats to their safety.

Outlaw said as the officers drove toward TJ and a 17-year-old, who were standing on the corner of 18th and Barbara, an officer turned on the car’s flashing lights, then heard gunfire. Outlaw said preliminary evidence showed the boy fired the shot into the car, which shattered a rear window, then flew through a passenger’s headrest and into the car’s headliner. Camacho was left with injuries in both eyes by shards of glass, police records show.

A lawyer for TJ’s father on Tuesday disputed the accusation that the child fired the gun, calling it “egregious speculation” that has not been confirmed by evidence.

It’s “unbelievable that [Outlaw] would refuse to provide factual evidence to the press about the details of the shooting that she has in her current possession, and then speculate egregiously as to whether or not my client had a gun in his hands, let alone fired one,” said the lawyer, J. Connor Corcoran.

The mother of the 17-year-old — who has not been charged — previously said her son’s account conflicted with how police described the event. She said her son told her the officers did not turn on their flashing lights or identify themselves as police officers before shots were fired. She did not respond Tuesday to requests for further comment.

Outlaw said police have evidence that the lights were on before a shot was fired into the car. She did not specify what that evidence was.

After TJ fired a shot, Outlaw said, two officers — identified in police records as Mendoza and Sarpong — got out, took chase, and each fired a shot back at TJ.

Sarpong then “maintained cover,” Outlaw said, while Mendoza continued chasing the boy and shot two more rounds, one of which fatally struck him in the upper right back and exited through his left chest.

Police sources told The Inquirer last week that investigators were examining whether TJ had tossed his gun before Mendoza fired the last two shots. Outlaw declined to offer specifics about where the boy’s gun was at the time he was shot, or how close Mendoza was to him.

Surveillance video obtained by The Inquirer appears to show the moments just after the shooting, with Mendoza standing on the sidewalk yelling: “Yo, I got one down here!”

Outlaw said police recovered a 9mm semiautomatic handgun from the scene, which she said TJ had been carrying. It was a stolen gun, she said, equipped with a laser and loaded with one round in the chamber and five rounds in the magazine.

Police Department policy says officers are not justified in using deadly force solely if a suspect resists arrest or attempts to escape. Other factors are supposed to be taken into consideration, such as whether a suspect was armed, or posed an immediate threat to an officer. Officers should not shoot at a fleeing suspect “who presents no immediate threat of death or serious bodily injury,” the policy states.

In 2017, then-Commissioner Richard Ross fired Officer Ryan Pownall for fatally shooting David Jones after Jones had tossed an illegally possessed gun while running away from a traffic stop. Ross said Pownall had been justified in firing some initial shots toward Jones, but that Pownall had violated department policy by failing to re-evaluate the situation once Jones was running away and unarmed.

“Just because you had an opportunity and you had the right to fire at one point during an encounter does not mean you have that same right to do so throughout the encounter,” Ross said then. “There’s an obligation to reassess the situation.”

Staff writers Barbara Laker, David Gambacorta, and news researcher Ryan W. Briggs contributed to this article.