The Special Investigations Unit (SIU) is probing a police shooting in the parking lot of a Mississauga big-box mall that sent one man to hospital on Tuesday night.

Undercover Peel officers were watching three men inside a stolen SUV when the incident occurred at the mall located at Winston Churchill Boulevard and Eglinton Avenue West at about 7 p.m., the SIU said.

When officers approached the men, a confrontation ensued, and an officer fired a single shot through the vehicle that struck one of the suspects in the shoulder, the SIU said.

The 23-year-old man was hit in the shoulder and taken to St. Michael's Hospital. His injury is not life-threatening.

One of the other suspects was arrested while the third man ran off.

The shooting shocked shoppers and employees in the area.

"All of a sudden we just heard a bang ... We weren't sure if it was a firework or something but we looked and all we saw was a guy in a car with a gun," 16-year-old Stephanie Kuhn, who was outside on a break during her McDonald's shift, told the Toronto Star.

A gunshot wound was visible in the windshield of the dark Chevy Trailblazer, but police wouldn't confirm that the man was shot when he refused to drop a handgun.

The SIU is a civilian agency that investigates police incidents resulting in civilian death, sexual assault or serious injury.