Two people are dead following a multi-car pileup near Sarnia, Ont.

The incident occurred just after 10 a.m. Thursday in the eastbound lane of Highway 402.

Multiple vehicles were involved, including a marked Ontario Provincial Police cruiser, which was reportedly responding to the scene of a prior motor vehicle collision in the area.

The Special Investigations Unit — the civilian agency responsible for investigating circumstances involving Ontario police and civilians that have resulted in a death, serious injury or allegations of sexual assault — is involved in the accident investigation because the cruiser was involved.

Dave Revington from London, Ont., told CBC News he was driving about 70 km/h when the visibility deteriorated. He began to slow down when he saw an ambulance, and while he was able to stop safely he was hit from behind by other cars and trucks

"People were out of their vehicles on the road and I had my window down and guys were shouting 'get out of the way, get out of here,' and guys ran into the ditch to the right and then you could hear the vehicles starting to pile up behind us," Revington said.

"You could hear the tractor-trailers hitting — bang, bang, bang, you couldn't see them for the snow but you could hear them."

Revington said he saw several people seriously injured. Those who weren't injured were taken to a community hall minutes from the highway where they're now waiting to see what happens next.

It's not clear how long the highway will be closed as the SIU and the OPP accident reconstruction team tries to figure out what happened.

The eastbound lane of the highway is closed from Nauvoo Road to Oil Heritage Road. And the westbound lane is closed from Nauvoo Road to the Forest exit.

Officers are on scene and the OPP has set up a mobile command post. Members of the media are being asked to stay 100 metres from the crash scene.

The SIU sent five investigators, including a collision reconstructionist, to the scene.

Prior to the crash, Environment Canada had issued a snow squall alert for the Sarnia area.