A badly worn bearing in the steering mechanism has become a prime suspect in last Wednesday's crash of a Virginia commuter bus that killed 11 people and injured 13.

National Transportation Safety Board officials theorized yesterday that the bearing -- actually a round tapered sleeve that held one end of a large tapered bolt -- had become so worn that the bolt slipped through the sleeve as if it had been unbuttoned. If that occurred, investigators said, it would have been impossible for driver Carl Earl to turn the bus to the left, back to the relative safety of Interstate 95.

Witnesses have told investigators that the bus veered to the right on the expressway, then plunged through a bridge guardrail and into Chopawamic Creek, a Potomac River tributary that runs through the Quantico Marine Corps Base 35 miles south of Washington.

Careful measurements have shown that the bus first struck the guardrail at a 6-degree angle 59 feet north of the bridge. The right wheels overrode the rail, then the left wheels, before the bus struck the bridge parapet and plunged itno the creek. The bus was estimated to be traveling about 60 miles per hour.

Paul Johnson, one of the surviving passengers, told The Washington Post that driver Earl was "wrestling with the steering wheel" before the bus plunged. "He would turn it a quarter-turn one way, then a quarter-turn the other way," Johnson said. "He was just struggling to straighten the bus out."

If the working theory is right, Earl had no chance to control the bus. He died in the crash along with 10 passengers. An autopsy determined earlier that Earl, 43, had not suffered a heart attack or other sudden disabling affliction. Test results announced late yesterday showed that he had not been drinking or smoking marijuana. The board is still waiting for a chemical analysis to determine if any prescription drugs were in his bloodstream.

One more important piece of evidence developed yesterday strengthened the worn-bearing theory. Board investigators found no evidence that there was anything wrong with the brakes. In fact, new brake linings had recently been installed on the rear wheels and the linings were adjusted to proper tolerances, board officials said.

However, the fact that the brake system appeared to be capable of functioning properly still leaves investigators with the nagging question of whether the brakes were applied. There were no skid marks on the highway. The team of specialists studying the remains of the bus in a hangar at Quantico continued last night to examine the braking system. Part of it ws badly damaged as a result of the impact of the crash.

The worn bearing is part of a ball-and-socket type joint that anchors the steering mechanism to the frame of the 22-year-old General Motors bus. There are two such bolts and bearings, one on each side of the bus. They are connected to a cylinder that controls the steering upon command from the steering column and wheel.

Two problems in the steering mechanism were discovered by investigators Friday and submitted to the safety board's laboratory for examination. One of them, a break at a major junction in the sterring system, was judged by metallurgist Dr. Michael Marx to have been caused by the accident. jThe other problem was wear in the suspect bearing.

Board member Patricia Goldman, in a briefing for the press, said that the owners of the bus, D&J Transportation of Fredericksburg, had no maintenance records on the bus but were not required by Virginia law to maintain such records. D&J operates solely within Virginia, and is therefore not subject to federal regulations Federal regulations require substantial record-keeping.

Goldman also said that the wind shield of the bus had been recovered and that no inspection sticker had been found. However, she said, "the windshield was in the water for 14 hours or so," so that the absence of the sticker did not necessarily mean the bus had not been inspected as state law requires. D&J officials have told reporters that the bus was properly inspected and carried a valid sticker.