Washington — Federal accident investigators called Thursday for a probe of the government agency charged with ensuring the safety of tour buses, saying its own look into four 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 tour bus industry “before crashes occur, not just after high-visibility events,” said Deborah Hersman, chairman of the National Transportation Safety Board.