Black Friday shoppers flee in horror as gunman, 21, is killed by police after he shot a 12-year-old girl and 18-year-old boy at Alabama mall

A Thanksgiving night shooting at Alabama's  largest enclosed mall  left a gunman dead and two people -- including a 12-year-old gi...

A Thanksgiving night shooting at Alabama's largest enclosed mall left a gunman dead and two people -- including a 12-year-old girl -- injured, authorities said, and 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.

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 also was shot once, but it's unclear by whom, Rector said.

Both of the injured were taken to hospitals. The girl has 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.

Witnesses described hearing gunshots inside the mall, which was open late on Thanksgiving. The scene was chaotic, with customers screaming and staff working to lock down the shopping center, witnesses told CNN affiliate WBRC.

The Hoover Police Department said the scene is now secure.

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.