A 27-year-old man is dead after a tractor-trailer rear-ended a vehicle on Highway 401 east of Toronto early Thursday morning.

The crash happened at approximately 1:20 a.m. when the tractor-trailer travelling westbound on Hwy. 401 near Lakeridge Road crashed into the vehicle for “unknown reasons,” the OPP said in a statement.

All westbound lanes of the highway from Brock Street to Salem Road were closed for hours, but have now been reopened.

Police said the male driver of the vehicle was transported to hospital with unknown injuries while his 27-year-old passenger was pronounced dead at the scene. The male driver of the tractor-trailer was not injured.

The province’s Special Investigation Unit has been called in.

The SIU is an arm’s length agency that probes incidents involving police and civilians that resulted in a serious injury, death or allegations of sexual assault.

With files from CP24 and The Canadian Press