WASHINGTON (AP) - Federal accident investigators called on Thursday for a probe of 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 other people 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 NTSB Chairman Deborah Hersman.

In one crash, federal inspectors gave a California tour bus company safety clearan ce 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 bus' brakes had failed.

Federal inspectors didn't 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 mec hanical 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 isn't supposed to be used when roads are slick because it can cause wheels to skid, NTSB said. A transmission retarder limits speed.

U.S. officials had previously fined bus operator Mi Joo Tour & Travel of Vancouver, Canada, for not testing drivers operating buses in the U.S. for drugs and alcohol. When the company failed to pay the $2,000 fine, federal officials order ed the company to cease U.S. operations. Mi Joo then paid the fine and was allowed in March 2012 - nine months before the crash - to resume transporting passengers in the U.S.

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 operating in the U.S. 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, president of Advocates for Highway and Auto Safety.

The motor carrier administration said in a statement that the number of unsafe companies and drivers the agency has taken off the road have more than tripled over the past three years through more comprehensive investigations.

"We have also brought together key safety, industry and enforcement organizations to ask for their help and support our efforts," the statement said. "We are continuously looking for new ways to make our investigation methods even more effective so we shut down unsafe companies before a crash occurs and will thoroughly review the NTSB's findings."

Tour and intercity buses carry about 700 million passengers a year, second only to domestic airlines, which move around 785 passengers annually.

The NTSB also posted online evidence gathered in two commercial truck crashes. In one crash near Elizabethtown, Ky., last March, a truck-tractor semitrailer rear-ended an SUV carrying eight people, pushing it into another car. The SUV exploded in flames. Six of the vehicle's occupants were killed and the other two were injured.

An investigation by Kentucky State Police and NTSB revealed the truck driver had been keepin g two sets of records on his hours behind the wheel, the board said. The records he gave police, which matched the truck company's records, indicated he was following federal regulations. The second set, found during a search, indicated he was driving for the 10th consecutive day on the day of the crash in violation federal regulations, the board said.

The trucking company was inspected by the motor carrier administration five days before the crash, but the inspection was narrowly focused and didn't include examining driver compliance with hours-of-service regulations.

The second truck crash occurred in June near Murfreesboro, Tenn. A tractor-trailer collided with eight other vehicles just past midnight. Two people in a 2003 Honda that overturned were killed. Six of the 13 people in the other vehicles were injured.

The truck driver had violated federal rules limiting the hours drivers can spend behind the wheel, and a review of the company's records found fo ur more drivers with similar violations.

The motor carrier administration knew of repeated hours-of-service violations by the company's drivers but conducted only a narrowly focused safety inspection in 2011 that allowed the company to continue operating.