Ontario’s Special Investigations Unit (SIU) is investigating after four people were injured in a single-vehicle collision in Oshawa on Sunday afternoon.

The crash happened in the eastbound lanes of Highway 401 at Stevenson Road.

The SIU said at about 3:30 p.m. an OPP officer was conducting speed enforcement on the 401 near Thickson Road in Whitby when a driver they’d tried to pull over fled.

“A short time later, the driver was involved in a single vehicle collision -- a rollover -- in the area of Stevenson Road in Oshawa,” the civilian agency said in a statement provided to CTV News Toronto.

The province’s police watchdog said that all of the occupants of the vehicle were taken to the hospital with serious, non life-threatening injuries. Two occupants went to St. Michael's Hospital, while the others two were taken to Sunnybrook Health Sciences Centre, they said.

In a post on X, the OPP’s Highway Safety Division said that the right lane and off ramps lanes of the eastbound 401 at Stevenson were closed until further notice due to a police investigation.

At around 6:30 p.m., they later that the civilian agency has invoked its mandate in connection with this collision.

At this time, the SIU, an arm’s length agency called in to investigate whenever police are involved in a death, serious injury, or allegations of sexual assault, has dedicated one subject official and two witness officials to this case.

More to come. This is a developing story.