A woman in her 40's was killed in a single-car rollover on Highway 400 in Barrie, Wednesday morning.

A car rolled over in the northbound lanes near Dunlop street, then burst into flames at approximately 1:25 a.m.

Ontario Provincial Police Sergeant Dave Woodford says they are trying to figure out what caused the fatal crash.

"We don't know why at this time, but the vehicle was traveling northbound and struck the bridge support at Dunlop Street then rolled numerous times into the centre lane of the highway, and immediately caught on fire and was fully engulfed in flames," say Woodford.

Woodford says the woman who was lone occupant of the vehicle was partially ejected and was assisted by several motorists who were passing by at the time of the crash. She was taken to hospital where she died from her injuries a short time later.

The highway was closed for just over six hours as police conducted their investigation.