BETHESDA, Md. -- A man is dead after a load of lumber fell on him from a highway overpass outside Washington, D.C.

Authorities in Bethesda, Md., said the man's BMW was crushed when a tractor-trailer overturned on the Capital Beltway Thursday morning and spilled its load of wood onto the highway below.

Fire officials said the man apparently died on impact, but his car was also hit by several vehicles. Three other people were taken to hospitals.

The truck was carrying wooden fencing from Canada to Norfolk, Va. About six vehicles were hit by the falling debris.

The cause of the crash is under investigation, but police said they believe neither speed nor weather was a factor. The load in the tractor-trailer may have shifted and caused the crash.

"Typically what happens is that the load shifts, the driver loses control and, unfortunately at this particular location, the debris from the truck instead of just spilling onto the roadway, it fell onto the lanes of the 270 spur," said Montgomery County Fire and Rescue spokesman Pete Piringer.