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(CNN) —  

A Thanksgiving night shooting at Alabama’s largest enclosed mall that left a gunman dead and two people – including a 12-year-old girl – injured sent terrified shoppers running for safety.

The incident at the Riverchase Galleria in the Birmingham suburb of Hoover started with a fight between an 18-year-old and the 21-year-old gunman, police said. The confrontation occurred in the mall shortly before 10 p.m. (11 p.m. ET), near the Footaction and JC Penney stores on the second level.

Police do not know how the confrontation began, but “we do not believe at this point the 18-year-old was armed,” Hoover police Capt. Gregg Rector said.

The gunman opened fire, shooting the teen twice in the torso, according to police.

An armed Hoover police officer who was working security at the mall confronted the gunman as he ran away from the scene and fatally shot him, police said.

A 12-year-old girl described by police as a bystander was shot once, but it’s unclear by whom, Rector said.

Both of the injured were taken to hospitals. The girl had surgery and “does not have life-threatening injuries,” Rector told CNN. Her condition was stable Friday morning, police said.

The injured 18-year-old was in serious condition when an ambulance took him to the hospital, police said.

The officer involved has been placed on administrative leave while the Jefferson County Sheriff’s Office investigates the incident, police said.

“We were fortunate that that Hoover officer was there where he needed to be, and we believe at this point that maybe he prevented further injuries,” Rector said.

’Freaking out’

Witnesses described hearing gunshots inside the mall, which was open late on Thanksgiving. The scene was chaotic, with customers screaming and staff working to help people flee or hide, witnesses told CNN affiliate WBRC.

“They (put) us in supply closets and locked the doors. And then we sat there for five to 10 minutes, all freaking out,” Lexie Joyner told WBRC. “And then they opened the escape-route doors, and we escaped through the escape-route doors.”

Hoover is about 10 miles south of Birmingham.

Brookfield Properties, which owns the shopping center, said it was devastated by the shooting.

“We are working closely with the Hoover Police Department and are grateful for their swift action to contain the situation,” spokeswoman Lindsay Kahn said.

The shopping center closed for the rest of the night, but reopened before sunrise for Black Friday shopping.

In an unrelated incident the next day, a male was shot in the wrist at a mall in Elizabeth, New Jersey, according to Mayor Chris Bollwage.

No one has been arrested, and Bollwage said the victim was not cooperating with police to identify the suspect.

CNN’s Jason Hanna and Dianne Gallagher contributed to this report.