Investigator Derrick Wide told CBC News that the SIU believes that gunfire first rang out in the early hours of Sunday morning, before police arrived at the scene.
"The information we've received at this point and time is that at about 2:51 a.m., occupants that were at a local nightclub were leaving the nightclub…and there was an altercation in the parking lot area," Wide said in an interview on Sunday.
Wide said gunfire ensued, which left two people wounded. A third person was injured, not by gunfire.
"This interaction was between groups of civilians and the police weren't involved at that time."
Wide said that officers who were patrolling nearby came to the scene, which led to an interaction with a vehicle that had three people inside.
"The officers discharged their firearms...and the vehicle left the scene," Wide said.
The vehicle came to a rest in a snowbank on Dufferin Street. Officers soon caught up with the vehicle.
"Two of the people in the front seat were taken to area hospitals and the third party in the back was taken into police custody," Wide said.
He said it is not known if the people in the car were involved in the initial incident.
Wide said the SIU is investigating witnesses and examining the scene. Police are probing the initial shooting incident.
A news release from the SIU said the initial alteraction that drew the attention of police occurred "outside the Muzik nightclub," located inside the Exhibition Grounds.
But Zlatko Starkovski, the nightclub's president, issued a statement Sunday morning saying the shooting happened "in a parking lot located on Exhibition Grounds" that is not "on the premises owned and operated" by Muzik.
The SIU investigates all incidents involving police in which death or serious injury occur, or when allegations of sexual assault are raised.