ZAOYANG, Hubei - A truck exploded while traveling on an expressway in Hubei province on Friday, killing at least three people and injuring four others, local police have reported.

The powerful blast caused a 5-meter-wide and 3.5-meter-deep hole in the road and damaged farm houses near the roadside. ?Pieces of the truck were blow 100 meters away in the explosion, according to witnesses.

The driver was among the killed. An old couple were injured after the explosion damaged the farmhouse they were in and have been sent to hospital, police said.

Investigators said they have yet to determine the cause of the explosion, though the truck was licenced to transport explosives.