ATLANTA (AP) _ A woman who failed to buckle her lap belt was decapitated by a shoulder harness in a traffic accident, a preliminary investigation shows.

The probe by Calspan, a research company contracted by the federal government, concluded that the shoulder harness on a 1988 Ford Escort cut through the neck of 25-year-old Sun Young Ham in the June 1 accident.

Mrs. Ham's death has raised concerns about safety belt systems in millions cars that combine a motorized shoulder belt and manually operated lap belt.

Consumer advocates contend that many motorists are lulled into a false sense of security and fail to buckle lap belts in cars with motorized shoulder harnesses.

Federal and county investigators have said they believe Mrs. Ham's chances of survival would have been greatly improved by wearing a lap belt.

Ford Motor Co. officials said they believe it's too early to reach conclusions.

The automaker's preliminary investigation suggests the shoulder belt had nothing to do with the decapitation, said spokesman Beryl Goldsweig.

Mrs. Ham, a resident of suburban Atlanta, was a passenger in the front seat of a car driven by her husband when it was broadsided in the rear quarter panel on the passenger side.

The impact threw her to the right, pressing her neck against the shoulder belt as she fell against the door and was thrown from the car, Calspan's report said.

Her husband survived the accident, even though he also failed to buckle his lap belt.

The National Transportation Safety Board and the National Highway Traffic Safety Administration are investigating the accident.