As new details emerged Thursday about the police killing of 12-year-old Thomas Siderio — the youngest person to have been fatally shot by a Philadelphia police officer in decades — there were conflicting accounts about the circumstances leading up to his death.

Police said four plainclothes officers were in an unmarked car around 7:20 p.m. Tuesday near 18th and Barbara Streets when a bullet went through their window. According to police records obtained by The Inquirer, the four officers were Edsaul Mendoza, Kwaku Sarpong, Robert Cucinelli, and Alexander Camacho.

The officers approached the boy and a 17-year-old, police said, because they believed one of them had a handgun. They said they turned on their flashing lights, then heard gunfire and a rear window shattered. Two officers got out, police said, and fired toward Thomas, who they said was holding a handgun.

One officer then briefly chased the boy and fatally shot him in the back, police said. The records did not indicate which officer fired the shot that struck and killed the child.

Kim Tomasetti, the mother of the 17-year-old — who was not charged — said her son’s account of the shooting conflicts with how police described what happened.

According to her son, she said, the officers did not turn on their emergency lights or identify themselves as police before any shots were fired. The boys, she said, were afraid that someone in the car was about to attack them and didn’t know there were officers inside.

Efforts to reach three of the officers were unsuccessful. Reached by phone Thursday, Mendoza declined to comment. The union that represents Philadelphia police officers declined to comment.

The four officers have been on the force for less than six years, according to city payroll records.

A spokesperson for the Police Department, citing department policy, said Thursday that officials could not confirm the names or assignments of the officers involved in the incident. Department policy typically allows officials to release the names of officers who shoot people within 72 hours of an incident, though there are exceptions if the department determines there are concerns for the officers’ safety.

Philadelphia police investigators and the District Attorney’s Office Special Investigations Unit are probing the shooting. A spokesperson for the District Attorney’s Office declined to comment, citing the ongoing investigation.

Kevin Lessard, a city spokesperson, said in a statement Thursday that “this is a deeply troubling incident with a tragic loss of life.” He added that the officers had been placed on administrative duty pending the outcome of the investigation, as is customary.

One of the officers had been involved in a fatal shooting in 2018. Cucinelli and a partner attempted to stop a man traveling the wrong way down the street on his bicycle near the 2000 block of McKean Street, records show. The man attempted to flee and allegedly fired seven rounds at the two police officers, who returned fire. The suspect later ran into two other officers, who fatally shot him, police said.

On Thursday, family and friends mourned Thomas, who was in seventh grade at George W. Sharswood Elementary School. Outside Barry Playground near the shooting, a shrine of candles sat burning in memory of Thomas, some bearing messages in permanent marker reading, “Love you, bro.”

Officials said Wednesday the four officers were detailed to a task force based in the city’s South Division and were conducting an ongoing investigation into illegal gun possession. Deputy Commissioner Benjamin Naish, who oversees investigations, said a social media post prompted the officers to go to that location, but he could not say who made the post.

When officers arrived, police said, they saw Thomas and a 17-year-old on bicycles and believed one of them had a handgun. They drove closer, police said, and turned on their emergency lights, then heard gunfire and the back passenger-side window shattered.

Camacho was injured in both eyes by shards of glass, police records show.

Police said two of the officers then got out of the vehicle and fired at Thomas, who they said was holding a gun and fled east on Barbara Street. One officer chased after him and fired twice, hitting him in the upper back area. The bullet exited through the boy’s chest, police said.

He was taken to Penn Presbyterian Medical Center and pronounced dead minutes later.

Naish said Wednesday that the fact that the boy was shot in the back doesn’t mean “that there was not a gun being pointed towards or in the vicinity of the officer.”

Police said investigators recovered a 9mm semiautomatic handgun Thomas was carrying. It was equipped with a laser and was loaded with one round in the chamber and five rounds in the magazine.

Investigators found five shell casings at the scene.

Video footage provided to The Inquirer by residents was motion-activated and depicts only the aftermath of the shooting. Naish said the officers were not equipped with body-worn cameras.

City officials denied a request to provide surveillance footage from cameras installed at a nearby city-operated playground, citing the ongoing investigation.

Philadelphia Police Department policy says an officer would not be justified in using deadly force solely if a suspect resisted arrest or attempted 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.

The U.S. Supreme Court, in the 1986 case Tennessee v. Garner, ruled that it was “constitutionally unreasonable” for police officers to use deadly force to stop a suspect who had committed a felony from escaping.

But under Pennsylvania law, officers are justified in using deadly force if a suspect “has committed or attempted a forcible felony, or is attempting to escape and possesses a deadly weapon.”

The District Attorney’s Office has argued the state Supreme Court should declare that portion of the law unconstitutional.

Tomasetti, the mother of the 17-year-old, said she believes the shooting could have been avoided if officers had identified themselves sooner.

”If you’re doing an investigation looking for something, you’re not going to be driving around with your lights on to announce yourself ‘Hey, kids, cops are here. We’re looking for you,’” she said. “You’re going to do it sneaky. But at the moment you stop in front of a person, you announce yourself. You wanna drive around without your lights, no problem. The minute you stop, you gotta identify yourself as a cop.”

Staff writers Ryan W. Briggs, David Gambacorta, and Oona Goodin-Smith contributed to this article.