WASHINGTON — Federal accident investigators called Thursday for an inquiry into the government agency charged with ensuring the safety of commercial vehicles, saying their own look into four tour bus and truck crashes that killed 25 people raises ‘‘serious questions’’ about how well the agency is doing its job.
Federal Motor Carrier Safety Administration inspectors failed to respond to red flags indicating significant safety problems on the part of bus and truck companies involved in accidents in California, Oregon, Kentucky, and Tennessee, documents released by the National Transportation Safety Board said. Besides those killed, 83 were injured in the crashes, many of them seriously.
The motor carrier administration needs to crack down on bad actors ‘‘before crashes occur, not just after high visibility events,’’ said the NTSB chairman, Deborah Hersman.
In one crash, federal inspectors gave a California tour bus company safety clearance a month before one of the company’s buses overturned near San Bernardino last February while returning from a ski resort. Seven passengers and a pickup truck driver were killed, 11 passengers were seriously injured and 22 others received minor to moderate injuries. The bus driver told passengers the brakes had failed.
Federal inspectors did not ask to examine Scapadas Magicas’ buses during their visit to the company’s headquarters near San Diego even though the company’s buses had been cited previously for a host of mechanical problems during spot roadside inspections.
California Highway Patrol crash investigators found a catastrophic failure of the brakes that a proper inspection by federal officials could have foreseen. All six brakes on the crashed bus were defective, according to the NTSB’s report. Drums were worn or cracked, linings were worn down and some were otherwise ‘‘defective or inoperative.’’
Two of the company’s other buses had serious mechanical defects, and the company had failed to have its buses regularly inspected by the state.
In another accident, a driver lost control on a slippery highway near Pendleton, Ore., in December 2012, sending his bus through a barrier and down a steep slope. Nine people were killed and the driver and 37 passengers were injured.
The driver of the bus had been on duty for 92 hours in the eight-day stretch before the accident, exceeding the 70-hour federal limit.
The bus was traveling too fast in poor weather, and the driver had the vehicle’s transmission retarder engaged even though it is not supposed to be used when roads are slick because it can cause wheels to skid, the NTSB said. A transmission retarder limits speed.
US officials had previously fined bus operator Mi Joo Tour & Travel of Vancouver, Canada, for not testing drivers operating buses in the United States for drugs and alcohol. When the company failed to pay the $2,000 fine, federal officials ordered the company to cease US operations. Mi Joo then paid the fine and was allowed in March 2012 — nine months before the crash — to resume transporting passengers in the United States.
Federal inspectors had given Mi Joo satisfactory safety rating in 2011 — a year and a half before the crash — even though an NTSB review afterward of those inspections revealed ‘‘longstanding and systemic’’ problems dating to when the company first began its US operations in 2007.
‘‘This fatal crash might have been prevented if the [motor carrier administration] had exercised more effective federal oversight’’ during the 2011 inspection, the NTSB said.
The NTSB’s findings are ‘‘very disturbing and, frankly, deadly for the public,’’ said Jacqueline Gillan, of Advocates for Highway and Auto Safety.
The motor carrier administration said the number of unsafe companies and drivers the agency has taken off the road have more than tripled during the past three years.