Sources tell ABC11 that an autopsy shows that 62-year-old Carolyn Ann Watkins died from blunt force trauma to the head and neck from injuries sustained in the crash on Swift Creek Road near Smithfield.
The medical examiner said Watkins' death would have been immediate.
The accident happened sometime Thursday night or Friday morning.
Trisha Hall and her daughter found the car and called 911. They said an off-duty trooper pulled up and took over from there. They are horrified by what has happened.
"I walked over and looked down and was a little nervous about it," said Trisha Hall, who called 911. "I didn't see anybody in the car. I pray to God that woman died on impact because you know, if something could have been done, it just adds to a tragedy that's already horrible."
The family said Watkins - a grandmother of four - was perfectly healthy, and they don't know what could have caused her to drive off the road. They said they suspect she may have been hidden by the car's air bags - which deployed in the crash.
Trooper M.D. Williams - a 9-year State Highway Patrol veteran - filed the crash report. His report said there was no driver at the scene.
Williams has been placed on administrative duty with pay pending the outcome of the probe.
The family plans to file a lawsuit.
Watkins' funeral will be held Saturday at 1 p.m. at St. John's Missionary Baptist Church in Maple Hill.
The SBI is investigating why Watkins' body was not found earlier.