Safety Board Faults Truck and Bus Oversight WASHINGTON - Federal safety officials missed or ignored warning signs before four deadly crashes involving heavy trucks or intercity buses in the last year, the National Transportation Safety Board said on Thursday. The government shut down some of the companies, but only after a total of 25 people were killed and 83 injured in the four accidents. Highway travel by car keeps getting safer, and the rate of heavy truck crashes has decreased, but statistics do not show a decrease in the number of bus fatalities. While the Transportation Department calculates the number of fatal accidents per 100 million miles driven for cars and trucks, it does not do the same calculation for intercity buses, because it does not have mileage figures available for them, a spokesman said. The board, an independent federal agency, said the four accidents raised "serious questions about the oversight of motor carrier operations" by the Transportation Department. The Federal Motor Carrier Safety Administration, the Transportation Department agency that regulates commercial truck and bus safety, did not dispute any of the safety board's assertions. But Anne S. Ferro, the administrator of the agency, said that it had shut down "well over 100" unsafe bus and truck operators since 2010, compared with only about one a year from 2000 to 2009, and that the agency began a comprehensive review of its own operations two years ago. Ms. Ferro also said that her agency had about 350 inspectors to cover 10,000 bus companies and more than half a million trucking firms. In the most recent of the four crashes cited by the safety board, in Murfreesboro, Tenn., on June 13, a truck operated by H&O Transport hit eight other vehicles that had slowed down because of an accident. It was dry and clear, but the truck rear-ended a 1999 Oldsmobile Alero, which erupted in a fireball. Two people in the car were killed; six people in other vehicles were injured. The truck was speeding and on cruise control, and the driver had worked more than 10 hours beyond the 70 hours drivers are allowed to work over eight consecutive days. The Federal Motor Carrier Safety Administration knew that the driver had a history of "hours-of-service" violations, and that the company had a history of such schedule violations, the safety board said. But the administration's last inspection before the crash had been limited, the board said. In another accident, a Mexican-owned bus descending a mountain road in the San Bernardino National Forest in California rear-ended a car, crossed into the opposite lane, struck an embankment and overturned, colliding with a pickup truck towing a trailer. Seven bus passengers and the driver of the pickup were killed; the bus driver and 11 passengers were seriously injured. The bus had "numerous mechanical problems," the board said, including that all six brakes were defective. A month earlier, federal inspectors learned of maintenance problems at the bus line through roadside spot-checks. The Motor Carrier Safety Administration said it had conducted a review and found that the line's performance was "satisfactory," but it did not inspect any buses, the safety board said, or review many business records, because the company was in Tijuana. After the crash, the Transportation Department ordered the bus line, Scapadas Magicas, off the road. The safety board, which has no regulatory authority, unanimously recommended more vigorous audits and follow-ups. "They need to crack down before crashes occur, not just after high-visibility events," the board chairwoman, Deborah A. P. Hersman, said in a statement.