The province's Special Investigations Unit is probing two deaths on Highway 402 on Thursday following a 25-vehicle pileup that included a police cruiser and two fire trucks.

The Sarnia collision scene closed the highway in the area as officials investigated the pile of cars, tanker trucks and emergency response vehicles that began crashing there shortly after 10 a.m. during whiteout conditions.

The crashes came in the eastbound lanes after snow began blowing across the stretch of highway east of Forrest Road, causing some cars to hit each other and others to skid into the ditch. One Ontario Provincial Police cruiser, two Warwick fire trucks and an emergency response vehicle were among those damaged, said a staff sergeant from the Lambton OPP.

The SIU is called to investigate when police officers are involved in incidents involving sexual assault, serious injury or death.

Police said at least four people were seriously injured and have been taken to hospital by ambulance. It is unclear if that number included the deceased. Attempts to use an air ambulance at the scene were thwarted by the weather conditions.

None of the rescue workers involved in the crashes was among the injured, police said.

Highway 402 was closed at Nauvoo Road in both directions while emergency crews work at the scene. The OPP expects it to remain closed into the evening, as they must complete their investigation and remove the vehicles.

Conditions are poor throughout southern Ontario, with visibility of less than a kilometre, in bursts of now, according to Environment Canada. The weather agency says winds are blowing at about 60 kilometres an hour, which can make for sudden and isolated visibility problems.

Crashes have also been reported in Chatham, and Highway 401 has been shut down in certain sections. London, Ont., has also closed some roads, as has Hamilton, where a section of the Lincoln Alexander Parkway has been shut down after multiple crashes.

Police are advising motorists throughout the region to drive with caution, or even better, just stay where they are.

"We are just encouraging motorists that if they can postpone their trip to do so," OPP Const. Aaron McPhail told The Canadian Press, saying those who choose to drive should make sure they have an emergency kit and plenty of gas.