Ontario's police watchdog is investigating a fatal shooting involving Peel Regional Police after a man believed to be about 30 years old was shot and killed early Saturday in Mississauga, Ont.

Jasbir Dhillon, spokesperson for the Special Investigations Unit, told reporters that Peel police were called to a "disturbance" at a residence in the city, west of Toronto, at about 12:30 a.m. ET. Peel police tweeted that officers responded to reports of a person with a weapon.

Dhillon said when officers arrived at the scene, the man had already left the residence. She said the man was later located a short distance west of the area of the original call.

"Upon the officers locating him, there was an interaction. One of the officers discharged his firearm. And we know that he was struck," she said. 

Jasbir Dhillon

Jasbir Dhillon, spokesperson for the Special Investigations Unit, speaks to reporters after a man was shot and killed by Peel Regional Police early Saturday. (CBC)

Dhillon said the man was transported with no vital signs to St. Michael's Hospital in Toronto. She said he was pronounced dead in hospital early Saturday.

The man has not been identified yet, she added. She said he is "roughly" 30 years old.

She said the shooting occurred outside a home on Torino Crescent, a small street in a residential area.

"Obviously, the investigation is in its preliminary stages. And we are looking at gathering more information, and obviously, speaking to as many witnesses as we can," she said.

Dhillon said the SIU has yet to determine why the man left the residence and was found a distance away. She declined to elaborate on the nature of the "disturbance" call and would not confirm if the man had a weapon.

She would not say how many shots were fired. 

SIU investigates Peel police shooting

A Special Investigations Unit officer takes a first look at the scene of a fatal shooting involving Peel Regional Police in Mississauga. The officer is in a driveway of a home. (Tony Smyth/CBC)

The SIU is expected to be at the scene for "a good part of the day," she said.

Steve Walker, superintendent of Peel Regional Paramedic Services, confirmed the person was in critical condition when taken to hospital. He could not confirm that the person suffered gunshot wounds.

The SIU has assigned six investigators and three forensic investigators to probe the incident, which was witnessed by five officers.

The SIU is urging anyone who may have information to call the lead investigator at 1-800-787-8529. Anyone who may have video evidence related to this incident is urged to upload the video through the SIU website.