Authorities have found 12 shell casings from the weapon of a Des Plaines police officer who fatally shot an alleged bank robber and accidentally wounded a bystander Tuesday on Chicago’s Northwest Side, according to a spokesman for the Chicago Police Department, which is investigating the incident.

The officer, whose name has not been released, was among the Des Plaines cops pursuing Christopher Terrell Willis, 32, into the city after an armed robbery and carjacking in the northwestern suburb.

During the chase, authorities said, Willis shot a Chicago officer in the head and then fled into UpBeat Music and Arts, a storefront school on Irving Park Road. They said the Des Plaines officer killed Willis and likely shot the bystander, a 15-year-old boy, inside the school.

News accounts have described the boy as getting hit in “crossfire” and a “shootout.”

But Willis, once inside the store, “did not have time to get a round off, as far as we can tell,” the CPD spokesman, Anthony Guglielmi, said.

“He was shot very quickly in the store” by the officer, Guglielmi said. “The boy walked right into the shooting.”

The officer fired the rounds from a semiautomatic rifle and “physically damaged” a weapon that Willis possessed, Guglielmi said.

In a briefing with reporters Wednesday, Chicago Police First Deputy Superintendent Anthony Riccio said Willis had pointed his gun at the Des Plaines officer.

No other officers were inside the music school, according to Guglielmi.

Willis died from multiple gunshot wounds, according to the Cook County medical examiner’s office.

The boy was hospitalized with gunshot wounds to his arm and chest, Riccio said.

The CPD investigation, which could take weeks, is examining the conduct of both Willis and the Des Plaines officer, according to Guglielmi.

Riccio, CPD’s second-in-command, did not wait for the outcome of that investigation to express his opinion.

“There’s no question in my mind that the Des Plaines officer acted properly,” Riccio said, according to the Sun-Times.

An Illinois law that took effect in January requires the Des Plaines Police Department to conduct a “thorough” internal review of the shooting. The law was passed after a WBEZ and Better Government Association investigation of 113 shootings by suburban Chicago police officers.

That probe found that the officers’ departments had done virtually no examination of how the shootings, which may have been justified, could have been avoided.

Des Plaines police officials did not respond to questions about the shooting or the review they are required to conduct.

Willis’s alleged accomplice, Maurice Murphy, 32, was arrested shortly after the bank heist and charged Friday in a federal criminal complaint with bank robbery.

Chip Mitchell reports out of WBEZ’s West Side studio about policing. Follow him at @ChipMitchell1.