The province's Special Investigations Unit (SIU) is probing the circumstances around an early-morning collision Tuesday that sent one boy to hospital and another into police custody after an SUV slammed into a tanker truck in the city's north end.

Police were called at 4:45 a.m. for reports of a collision at Finch Avenue West and Dufferin Street.

Police said two 13-year-olds were in an SUV that belongs to a family member headed southbound on Dufferin. The SUV hit the side of a tanker truck that was making a left turn at Finch, Sgt. Murray Campbell of the Toronto police traffic services division told CBC Toronto.

The SUV then spun out of control and slammed into a GO Transit bus, Campbell later told reporters at the scene.

"It is believed that they were travelling at a very high rate of speed when this occurred," Campbell said, adding that witnesses have told police the SUV ran red lights as it sped south on Dufferin.

The driver was not injured and has been placed under arrest, Campbell said, but he could not indicate what, if any, charges he might face. The passenger was taken to hospital with a "serious, life-altering" leg injury, he said.

"It could have been absolutely catastrophic," Campbell told CBC. "Two inexperienced people, youths, in a car travelling at what's believed to be a high rate of speed, there could have been a lot more vehicles involved and more serious injury or death."

The SIU issued a news release later Tuesday to say that three investigators and two forensic investigators have been assigned to probe the collision. According to the release, a York Regional Police officer spotted the youths in the SUV near Dufferin and Centre Street in Vaughan around 4:40 a.m.

The officer attempted to stop them, but they fled. The collision occurred a short time later.

The SIU probes incidents involving police that result in a civilian's injury or death, or allegations of sexual assault.

The Toronto police collision reconstruction unit will probe the crash. (Ali Chiasson/CBC)

A man who arrived at the scene late Tuesday morning saying he was the father of the passenger said he has yet to see his son. Doctors have told his wife that they were trying to save his son's leg.

He said the SUV belongs to him — he had left the key in his jacket pocket for the first time. He did not give his son permission to drive the vehicle, he said. The teens drove to Dufferin and Finch from the Jane and Finch area, he said.

Images from the scene showed the crumpled SUV resting against a double-decker GO bus, which was on its way to the nearby GO Transit bus terminal when it was hit, Metrolinx spokesperson Anne Marie Aikins said. The bus did not have passengers and the driver was uninjured, she told CBC Toronto.

The bus driver remained at the scene and will assist with the investigation, Aikins said.

The tanker truck was also empty and the driver was not hurt, Campbell said.

The intersection was closed as investigators gathered evidence. Campbell urged anyone with dash-cam footage or other surveillance video of the area at the time of the incident to contact police.

Buses in the area had to be re-routed as a result of the collision, including the 36 Finch West, the 939 Finch Express, the 105 Dufferin North and the 104 Faywood.