Two people are in hospital, one with life-threatening injuries after a tractor trailer rear-ended another on the 401 late Friday afternoon.
It happened in the eastbound lanes near Iona Road around 5:45 p.m.
Police say the driver and a passenger of the truck that rear-ended the other sustained life-threatening injuries.
Major delays ensued as a detour was put into place.
The eastbound lanes of the highway were closed until early Saturday morning for cleanup and the investigation.
Police continue to probe the incident.