Ontario Provincial Police are still investigating what caused an SUV to plow into several cars and fly through the air on the 403 near Waterdown Road Monday evening, snarling traffic for hours and sending two people to hospital in critical condition.

“It’s unknown yet because we’re examining all factors,” Const. Graham Williamson told CBC Hamilton. “One possibility that is being looked at is a medical condition.”

The incident happened around 6:30 p.m., when five vehicles — including a tow truck and a police officer – were stopped on the shoulder of the 403’s westbound lanes for a “relatively minor collision,” Williamson said.

A black Cadillac Escalade then plowed into the five vehicles on the shoulder and then into two other cars on the roadway. The 44-year-old Ancaster driver of the SUV and a 59-year-old Burlington woman who was in one of the vehicles were both taken to hospital with life threatening injuries.

Three children — two of whom were visiting the area from Holland — were also taken to hospital with non-life threatening injuries. A Toyota Yaris that was involved in the incident was completely totaled, and could be seen by drivers passing by.

“Speed has not yet been determined as a factor,” Williamson said.

Traffic was snarled for several hours because of the crash, and many GO Transit buses were delayed up to an hour. The roadway opened again around 10:10 p.m.