Investigators are looking into what caused a serious crash on Highway 416 at Brophy Drive Saturday night.
Paramedics say a total of eight people had to be rushed to hospital.
Three of the patiens are said to have the most serious injuries but their conditions are being described as stable.
Ontario Provincial Police first sent notice of the road closure between Brophy Drive and Roger Stevens Drive at about 8:10 p.m.
One lane reopened in each direction just before 9 p.m.