National Transportation Safety Board investigators yesterday discovered a break in the steering mechanish of the commuter bus that crashed on Interstate 95 in Virginia Wednesday and killed 11 persons.

Laboratory tests were ordered to determine if the fracture could have caused the crash. The investigators emphasized that the mechanism could just as easily have been broken as a result of the accident.

The bus, carrying 24 persons, plunged through a bridge guardrail and traveled 80 feet horizontally and 25 feet vertically before crashing into a boulder at the bottom of Chopawamsic Creek about 35 miles south of Washington. Thirteen persons were injured.

The fracture in the steering mechanism, investigators said, occurred at the point where a complex box of gears and bearings connects the steering column with the front axle.

Safety Board member Patricia Goldman said the broken steering components were taken to the board's laboratories in Southwest Washington for detailed metallurgical examinations, which takes several days.

The fracture was discovered as investigators working at Quantico Marine Corps Base began to tear apart and examine parts of the 22-year-old General Motors bus. The broken steering mechanism was the only significant finding yesterday, but much work remains to be done, including a major investigation of the bus' braking system.

No skid marks, which would indicate an emergency braking attempt, have been discovered. "Even if the steering is a problem, it seems logical the driver would have tried to brake," one board official said. "So we're still left with a mystery."

The driver, 43-year-old Carl Earl, died in the accident. An autopsy ruled out a heart attack or sudden incapacitation. It will be next week before the board knows the results of tests to ascertain whether the driver, a college student employed by D&J Transportation of Fredericksburg, had been drinking or using drugs.

Is it possible the driver fell asleep?

"For all I know, he dropped a pack of cigarettes and leaned over to pick them up," a board official said. "We've seen that kind of thing. Or people play with a tape deck, lose concentration and get their head below the windshield. By the time they bring their head up, it's slam, right into the tree."