VAUGHAN, Ont. -- Ontario's police watchdog is investigating after a police cruiser parked on the shoulder of a highway was rear-ended, sending a man in the back seat to hospital.

The Special Investigations Unit says the incident began at about noon on Thursday, when a York regional police officer pulled over a vehicle on the highway in Vaughan, Ont.

The agency says the driver was asked to sit in the back seat of the officer's cruiser.

A short time later, the cruiser was rear-ended by another vehicle, and the man detained in the back seat was badly injured.

The SIU says the person driving the other vehicle was also taken to hospital with minor injuries.

The unit is called in to investigate incidents involving police where someone is seriously injured, killed or alleges sexual assault.