Anchorage police are investigating a vehicle crash that closed the Seward Highway for nearly six hours after a driver reportedly smashed into the back of a tractor-trailer. 

According to a statement from APD, a male driver of a 1997 Ford Taurus was heading southbound on the Seward Highway near Milepost 77.5 when is struck a 2014 International Harvester tractor-trailer that was stopped a construction zone.

"The tractor trailer was stopped at a construction zone per the direction of construction workers and rear ended by the Taurus," police wrote. "The workers had one lane closed and were operating a pilot car for traffic."

Following the collision, the driver of the Taurus was pinned inside the car with his lower body trapped. After emergency crews responded, he was flown to a local hospital with life-threatening injuries. 

The driver of the trailer was not hurt. 

Drugs, alcohol and speed have not been ruled out as factors, according to APD spokesperson MJ Thim. 

APD initially sent out a Nixle alert just before 11 p.m. on Wednesday of the road closures on the north and southbound lanes of the highway. Both lanes reopened at 4:50 a.m. Thursday. 

The investigation is ongoing.