Officials investigating the bus crash that killed 11 people and injured 13 Wednesday near Quantico have ruled out a heart attack or sudden incapacitation of the bus driver as the cause but are confronted with a number of other mysteries.

"We have no explanation of why the bus left the road," National Transportation Safety Board member Patricia Goldman said yesterday. "There are many, many possibilities."

Goldman said investigators have been unable to obtain the maintenance records for the bus, which was owned and operated by D&J Transportation of Fredericksburg. "That doesn't mean there are no records," a safety board spokesman said, "just that we haven't been able to locate them." Investigators also have been unable to find a section of windshield where a Virginia state inspection sticker normally would appear, she said.

Goldman said the accident raises the question of whether the board should study safety questions surrounding the growing bus fleet that has sprung up nationwide for commuters who travel long distances. Many small bus companies are wellrun operations, officials said, but they are subject to little regulation and rarely receive rigorous safety inspections required of larger fleets. At least 9,000 Washington commuters daily ride non-Metro buses operated by about 30 different companies.

The accident Wednesday involved one of nine buses owned by D&J Transportation of Fredericksburg. The bus, carrying 24 people, was returning to Fredericksburg from a building in Alexandria that houses Army offices.

D&J's headquarters is the residence of the company's owners, Doris Whitaker Hall and her husband John Way. Way said yesterday, "I've no idea what the problem was. That bus was worked on just this past Monday, the holiday. I greased it, checked out the brakes, and did a general checkout myself."

Way said the bus bore a current Virginia State Corporation Commission permit and a current inspection sticker. "If you could find the windshield you could tell what month" the inspection had been made, he said.

Divers yesterday searched the bottom of the 2-foot-deep Chopawamsic Creek, a Potomac River tributary where the bus came to rest at 4:36 p.m. Wednesday after plunging 40 feet from Interstate 95, near Quantico at the Prince William-Stafford county line.

The driver, 43-year-old Carl Earl, died of multiple injuries. The Fairfax County medical examiner performed an autopsy yesterday and told safety board specialists that there was no evidence of heart attack or other sudden incapacitation. Tests were continuing to determine any possibility of drug or alcohol use, Goldman said. Autopsies performed yesterday revealed no indication of carbon monoxide poisoning in any of the victims, Goldman said.

Virginia state trooper Stanley Gregg said that his preliminary investigation showed no skid marks and "nothing to indicate any braking." He said nothing of a suspicious nature had been found during preliminary mechanical inspections. Safety board specialists said it may be weeks before they determine precisely what caused the crash.

Virginia officials said the accident was the second worst bus crash in the state's history. Fourteen people were killed in an accident at Hopewell in 1935. Buses generally have an excellent safety record. Thirty-nine people, including 17 on school buses, were killed in 1979, the last year for which national figures are available. There had been seven fatalities on buses in Maryland, Virginia and the District of Columbia combined since 1975 before Wednesday's accident.

The D&J bus involved was a 1959 GMC coach. Way said the engine had been overhauled last spring and called the bus "as good as a 1975 or 1979 model. It was immaculate." He said the bus met all state safety requirements, including semiannual inspection.

The bus companies providing commuter service in the Washington area range in size from Greyhound and Trailways to little operations with only one or two buses. Some are big enough to demand attention from the U.S. and state regulatory agencies; others go unnoticed.

"It's easy for us not to even know about an operation, particularly if it is a charter," said W. R. Fiste of the Bureau of Motor Carrier Safety in the Federal Highway Administration.

Many commuter buses are chartered and many times a member of the charter pool also will be the bus driver, as was Earl. Way said he was a student at Northern Virginia Community College in Alexandria, where he parked the bus during the day.

D&J was licensed to operate only in Virginia and therefore was not subject to federal safety regulations or inspection. The Virginia State Corporation Commission's motor carrier operations division inspects and regulates buses hauling regular ticket-buying passengers, but SCC officials said their only authority with firms such as D&J is to issue a permit. The only requirement is that the companies have liability insurance.

Virginia law requires only that drivers for such firms have chauffeur's licenses, which require no special training. Both state and federal officials said they were disturbed that moonlighting workers are permitted to drive buses.

"A man works eight hours at his regular job with the regular pressures and responsibilities and comes out to drive a bus with an entirely new set of pressures and responsibilities," said federal inspector Clyde Williams. "Right now iths permissible but the [U.S. Department of Tranportation] is giving it a lot of study."

Paul Conn, transportation supervisor for the Maryland Public Service Commission, said, "we try to inspect- . . . [buses] once a year, but if they don't register with us we don't know about them . . . Of course, we can't inspect them all."

Small bus companies, he said, "are a lot different than Greyhound." Some small companies, he said, "try to get every ounce of brake shoe they can. They try to cut corners because they don't have the money that the big carriers have."