A tractor-trailer ran off a Prince William County roadway and overturned Saturday morning, killing the driver, police said.

The incident occurred about 10:30 a.m. at Russell Road and the I-95 interchange, near Quantico, police said, but they did not know yet why it happened.

The driver — who police did not immediately identify — was killed, authorities said. Police said to expect delays in the area while investigators probe the crash.