A federal official said yesterday that a bulldozer operator's mistake caused a gasoline pipeline rupture in western Fairfax County Thursday that forced the evacuation of hundreds of homes and an elementary school.

"There seems to be no other conclusion that anybody could reach other than it was a mistake on his part," said Richard L. Beam, director of the Transportation Department's office of pipeline safety. "Probably he didn't realize how deep he had the blade" of the bulldozer.

County officials and other authorities also were focusing their investigation on whether F.E. Gregory & Sons Inc., the bulldozer operators, had properly notified the pipeline owner that work was planned in the area. Notification is required by state and local laws so utility companies can clearly mark underground lines and monitor construction work. A spokesman for F.E. Gregory said, "Because of the seriousness of the question and the accident yesterday . . . we're not making any statements."

Scores of federal, state and local officials, as well as the utility company involved, directed a massive clean-up yesterday that has already cost more than $100,000 in firefighting supplies alone.

The leak sent thousands of gallons of premium unleaded fuel gushing skyward in Centreville shortly before 10 a.m. Thursday. The ground was drenched and nearby residences were soaked. There were no serious injuries, primarily because the gasoline never caught fire.

Nearly 50 families were still unable yesterday to return to their houses in Singleton's Grove, an unfinished town house development that was doused by the spewing gasoline. Local authorities helped the residents collect essential items from their homes. Pets were retrieved and plans were made to tow cars to safety, away from the fumes that wafted through the neighborhood.

Electricity had not been turned back on and would not be, fire officials said, until machines monitoring the highly explosive gasoline vapors showed a reading of zero.

Colonial Pipeline Co., owners of the pipeline, worked to replace the segment of its 32-inch main that had been gouged by the bulldozer's blade. The firm said it hoped to resume the flow of gasoline by early this morning.

County officials reported that some gasoline -- perhaps several hundred gallons -- had seeped into Little Rocky Run, a stream that flows indirectly to the Occoquan Reservoir, which supplies drinking water to more than 600,000 Northern Virginians. The officials said no apparent threat was posed to the water, however, and the gasoline could easily be filtered or chemically absorbed.

"We don't expect any large quantities that we can't handle," said Fred C. Griffith, engineering director at the Fairfax County Water Authority. "But we will monitor it closely."

Many of the displaced families took advantage of an offer by US Home, developers of the project, and with their few belongings checked into the Fair Oaks Holiday Inn, about five miles away.

"The whole neighborhood is here," said Barbara Beard, one of the displaced residents. "This is inconvenient but it could have been worse."

Investigators based their preliminary conclusion that the bulldozer operator, identified Thursday as Keith Carpenter, 37, was at fault largely because Colonial's pipeline appeared to have been buried 36 inches beneath the ground -- deeper than the 30-inch minimum required by federal regulations. Moreover, they said, ample markers in the area should have alerted workers to the location of the underground pipe.

"From everything we can see on the surface, they {the utility} appear to be in good shape," said Beam, head of the pipeline safety agency. "It certainly complied with any {regulations} we had."

According to Beam, the bulldozer operator had contended that he had not contacted the utility before the job Thursday as he had on other occasions because he "said he knew where the pipeline was."

Jerry Paisley, area manager for Colonial, told The Washington Post that F. E. Gregory, the construction firm, did not notify the pipeline firm before the bulldozing started.

"They've been very cooperative in the past," he said. "I don't know why they were working there without contacting us."

One industry source, who asked not to be identified, told The Post that the construction firm notified Colonial June 4 that construction was to begin near the gasoline line Monday. But the source said it was unclear whether the accident three days later occurred in the same area.

Paisley confirmed the June 4 notification, but said it concerned a sewer project that was aborted -- not the work that was under way Thursday.

Officials stressed that the mishap is another reminder of the potential hazards posed by the subterranean network of pipelines running throughout the Washington area.

"There are all kinds of things under the ground," said Richard A. King, deputy county executive in Fairfax for public safety. "It's incumbent upon people to be perceptive to the existence of these things, or else you pay the ultimate penalty."

Bruce Leinberger, president of the local division of US Home, the developers, said the firm would pay for accommodations and food for the displaced residents at the Holiday Inn through the weekend.

The hotel took on an unusually lived-in atmosphere. Bassett hounds and huskies, teen-agers and their parents wandered in the hallways and lobby.

Sixteen-year-old Mike Boone was wearing his braces and his Walkman yesterday as he bounded into an elevator. "I really don't feel inconvenienced at all," he said. "I'm comfortable."