Another two seconds, another 125 feet, and the commuter bus probably would have rolled to safety. But time and distance had run out. Although the driver wrestled desperately with the steering wheel, his bus plunged off a bridge and into the Chopawamsic Creek, 35 miles south of Washington.

The driver and 10 passengers were killed; the other 13 passengers were injured.

Eighteen days after the second-worst bus crash in Virginia's history, investigators are not certain why it happened. They are turning face up all the pieces they have found for their puzzle, and some pieces are still missing. t

Ten experts from the National Transportation Safety Board and four Virginia State Police troopers have interviewed at least three dozen witnesses and survivors. They have taken sections of the bus apart one bolt at a time, with emphasis on the steering and brakes, and submitted many parts to detailed metallurgical examinations.

They have studied and restudied the contours of Rts. I-95 to see if something there would explain the accident, and they have studied and restudied the last 48 hours of driver Carl Earl's life to find out if they contain any clues. Their interest ranges from Earl's sleeping habits to the history of his right thumb, which once was injured in an accident.

There has been one major discovery: wear in a vital component of the steering system on the 22-year-old bus was sufficient to have caused the accident, even if it didn't. The problem is almost impossible to detect through normal inspection.

"Had the [safety board] not gotten involved, we would have finished in four or five days because of lack of knowledge," said Virginia State Trooper Rick Keevil. "We would have found the obvious separation in the steering system, but the question then becomes, how far do you go?"

The state police would not worry themselves with questions such as these: How many of the more than 5,000 General Motors buses like the one that crashed are still running? Where are they now? The last bus was built in 1960 and there is no federal register to locate them.

Nobody knows how many thousands of people nationwide ride to work evry day in buses long since discarded by Greyhound and Trailways and picked up years later by small companies such as D & J Transportation of Fredericksburg, which owned this bus. In the Washington area alone, more than 9,000 commuters daily use buses other than Metro's to get to jobs inside the Capital Beltway.

The task of sorting out this mystery began for the safety board when Tom Calderwood, the investigator in charge, arrived on the scene four hours after the accident. By then, salvagers had dropped the bus at least twice, so considerable damage could have been caused that had nothing to do with the accident. The board has scrounged photographs from newspapers and other sources to show it exactly what the bus looked like before rescue and salvage began.

Experts have disassembled many components of the bus and run countless laboratory examinations on various parts.

Did a tire blow out? No.

What gear was the bus in? High gear, meaning the driver probably did not try to down-shift.

How fast was it going? Metallurgical tests may indicate the speed because a speedometer needle often leaves a microscopically detectable indentation in the face of the speedometer when there is impact.

The brake system, badly damaged in the crash, was partially rebuilt. New linings had been installed shortly before the accident and the brakes apparently were in great shape.

Two witnesses trailing the bus did not see brake lights. Did the lights work? Yes. All the bulbs were recovered unbroken and functioning. The filaments in the bulbs were not stretched, indicating that the brakes were not applied, because electrified filaments stretch upon impact.

Two major breaks were discovered in the steering on the crashed bus. One was clearly the result of the impact when the bus struck a huge boulder in the creek. The other was in a ball-and-socket component, known technically as a ball joint. It came apart, and if it happened before the bus left the highway, it would clearly have affected the steering.

Another D & J bus, just like the one that crashed, was put on a hoist so the experts could see what would happen if that ball joint came apart. At first, nothing. But if some parts were rattling around, it appeared possible that one part could lock the steering in a way that would prevent a left turn while the wheels were still carrying the bus gradually to the right, into the guardrail and the bridge. More testing is scheduled.

It has also been learned that General Motors changed the design of its ball joints some time after the construction of this bus. Why?, investigators are asking. A General Motors spokesman declined to comment when asked by The Washington Post.

Thus, it is possible that Earl was nothing more than a passenger on a doomed vehicle and that nothing he tried to do could help.

It is also possible Earl was a victim of his own inattention, another in an endless list of drivers who have looked the wrong way at the wrong second, during which span their vehicles travel 88 feet (at 60 mph) and everything they knew the last time they looked had changed.

Experts estimate that Earl had at the most five seconds, at the least one second to analyze and solve his problem as the bus moved from the left-hand lane across two more lanes and into the guardrail. Another 125 feet and the bus would have rolled safely into soft dirt and sand of a construction area.

Gale Braden, one of the board investigators, asked, "What would be going through a person's mind driving a bus? He's in traffic. If he has lost steering, he doesn't know if he lost it totally or partially. At what point does a guy decide to apply the brakes?

"If you've got all the facts, then the accident is solved, it should be very obvious. If you don't have all the facts you may come up with other solutions that are incorrect but supported by the facts that you have."

The bus itself was first sold by General Motors to a Trailways subsidiary in Florida. Since then it has had several owners. There are no maintenance records of the inspection have been found in a Fredericksburg garage. The bus was legal.

The bus had been well maintained by D & J, and John Way (the J of D & J) worked on it on weedends.

"I spent four days out there looking at parts of that bus," one safety board expert said, "and i never saw a thing I could attribute to carelessness. bThe brakes had just been relined and properly adjusted and there was grease everywhere." What about the ball joint? "You can see the joint from under the bus, but you can't see the wear without taking it apart," another safety board expert said.

The stretch of Rte. I-95 on which the accident occurred has been examined for potholes, slight bumps, crevices, cracks. The bus struck the guardrail at a slight angle (6 degrees) and overrode it. The rail was installed in 1962 at a height of 24 inches, the standard of the time. Dirt buildup at the base of the rail was 3 inches deep, giving it an effective height of 21 inches. There is no way the rail was going to stop a 24,000-pound bus, even if the rail had been at the new standard height of 27 inches.

An autopsy was performed on each victim to see if there was evidence of carbon monoxide poisoning (there was none) and to determine the nature of fatal injuries so recommendations can be made on the crash-worthiness of bus interiors.

Driver Earl's autopsy showed that he did not have a heart attack and had not been drinking or using marijuana. The contents of his stomach are being checked to see if they include orange juice. A crumpled juice container was found in the front section of the bus. Did Earl take a swallow of juice and shift his attention from the highway for a crucial instant? Nobody knows.

Earl's friends and relatives have been interviewed to determine if he was getting enough sleep and what he had been doing in the 48 hours before the bus hurtled off the bridge, at 4:35 p.m. on Feb. 18. There is nothing to indicate anything other than a normal, healthy schedule.

Earl, 43, picked up his passengers in Fredericksburg, drove them to an Alexandria office building, took his bus with him to Northern Virginia Community College in Woodbridge, where he was studying accounting, and then returned to Alexandria for his passengers late in the day. He drove the same bus every day, so was totally familiar with it.

Earl injured his right thumb driving a truck a few years ago. Could that injury have affected his ability to steer? The experts are checking.

This is just one of about 20 highway acidents a year the safety board choses to investigate, and board members and staff are frank about which ones they select and why. "we still work on a gross body count, because it is spectacular," said board member Patricia Goldman.

The timing was right: The D & J crash occurred close to board headquarters just a week before Chairman James B. King was to testify about the board's budget on Capitol Hill.

"The publicity is very important," Goldman said, "because our end result is serving safety and [publicity] makes people aware of the problems and solutions. It makes Congress aware, too." The board has no authority to issue regulations or to enforce them, a fact that surprises many. It investigates and recommends actions and depends on public opinion to force change.

By taking just the spectacular incidents -- which means most airplane crashes and only the biggest highway accidents -- the board opens itself to some criticism. "We have 95 percent of the fatalities and 3 percent of the resources," said Anthony L. Schmieg, director of the board's highway accident division. "Aviation has 3 percent of the fatalities and 95 percent of the resources." Last year, for example, more than 50,000 people died in airliner accidents.

Calderwood, a former Utah highway patrolman, has seen too many accidents like this. "I was in a terrible state two or three years ago," he said, "because it occurred to me that I was making my living on the misfortune of others. I finally reconciled myself to that because we are solving problems that might save other lives."

Calderwood shook his head. He had worked almost nonstop since the evening of the accident; he looks tired and migraine headaches, an old problem, have returned.

"Every time I go to work, it's a tragedy," he said.