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WASHINGTON (AP) - Federal accident investigators called Thursday for a probe of the government agency charged with ensuring the safety of commercial vehicles, saying its 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.

The Federal Motor Carrier Safety Administration needs to crack down on bad actors in the commercial-vehicle industry "before crashes occur, not just after high visibility events," said Deborah Hersman, chairman of the National Transportation Safety Board.

NTSB's look at deadly crashes in California, Oregon, Kentucky and Tennessee over the past year found serious weaknesses in oversight. The NTSB says that in each case, the motor carrier safety agency identified problems and "red flags," but didn't take action until after the crashes.