MISSISSAUGA, Ont. -- The province's police watchdog says a York regional police officer who pursued a speeding vehicle that crashed into a ditch, killing the driver, will not face criminal charges.

The Special Investigations Unit says its investigation included a detailed collision reconstruction analysis, a review of the in-camera video from the officer's cruiser, and video captured by cameras in the area of the incident.

The SIU says the incident began in Whitchurch-Stouffville, Ont., early on Dec. 20, 2015, when the officer spotted a speeding car and started to follow the vehicle.

After the vehicle made a sharp turn onto Highway 48, the officer activated his emergency lights and the SIU says the cruiser reached speeds of 145 km/h, but was unable to catch up to the vehicle.

The agency says the officer lost sight of the vehicle as it crested a hill, but after crossing the peak of the hill, he watched as it lost control, crossed two lanes of the road, rolled, and slammed into an embankment, killing the driver.

SIU director Tony Loparco says the officer did "absolutely nothing" to contribute to the cause of the collision and maintained a substantial distance, which is evident on the in-car camera video.

Accident reconstruction analysis concluded that the man failed to negotiate a left curve before his vehicle travelled approximately 70 metres across two opposite lanes of traffic and then struck a utility pole, the SIU said.

This launched the car into the air, after which it slammed into a dirt embankment in a ditch and the man, who was not wearing a seatbelt, was ejected from the vehicle.

"The man was already driving at a dangerous speed before the subject officer even activated his roof lights," Loparco said Tuesday in a release.

The SIU is an arm's length agency that investigates reports involving police where there has been death, serious injury or allegations of sexual assault.