CLARKSVILLE, Tenn. -- The Tennessee Highway Patrol said Saturday it's still not clear what caused the Goodlettsville couple's car to veer off the road late Friday night on Interstate 24 east in Clarksville.

Investigators with the Tennessee Highway Patrol said driver Kenneth Helms hit the rumble strip just before midnight near Exit 11. Helms ran off the road, and slammed into a tree. Helms' wife, Linda, was killed.

THP officials said other than skid marks on the right side of the interstate near mile marker 10, there are few clues to tell police what caused Helms, 67, to leave the roadway.

Speed and alcohol were not a factor, investigators said, and both victims were wearing their seat belts.