The accident also caused two serious injuries that were transported by ambulance to the hospital.
According to the Ontario Provincial Police, five trucks and two cars were hit in the pile-up, which occurred around 1:25.
— megan (@megwilkinson_) May 11, 2017
The police reported that after the impact, one of the cars was destroyed by a fire. The four victims were on board.
One of the trucks involved in the accident belonged to the GardaWorld security and transport company.
Another truck from the company went to the site late in the morning to retrieve what it was carrying.
An investigation is underway to determine the cause of the pile-up.
Highway 401 remains closed to the west at Joyceville Road. The fast lanes could reopen only Thursday night, police said.