U.S. transportation safety investigators are probing a series of incidents in which self-driving cars operated by Alphabet Inc.’s Waymo unit drove improperly near parked school buses in Austin, Texas.

The National Transportation Safety Board will “examine the interaction between Waymo vehicles and school buses stopped for loading and unloading students,” the agency said in a social media post.

The probe increases federal scrutiny on Waymo after a series of incidents in which its vehicles failed to fully slow or stop for school buses. The issue had occurred about two dozen times as of earlier this month, according to the Austin Independent School District. The U.S. National Highway Traffic Safety Administration, the country’s auto safety regulator, opened its own investigation last fall.

None of the incidents involved collisions and the company is confident that its safety around school buses is superior to human drivers, Mauricio Peña, Waymo’s chief safety officer, said in a statement.

“We see this as an opportunity to provide the NTSB with transparent insights into our safety-first approach,” he said.

The NTSB has no safety enforcement authority, though its probes lead to recommendations that are typically adopted by transportation companies such as airlines and manufacturers.

In December, Waymo said it would file a voluntary software recall to address the behavior. Waymo also said it had implemented software updates by Nov. 17 that it maintains have “meaningfully improved” vehicle performance around school buses.

Waymo has said it met with the Austin school district in December to review “data collection of various light patterns and conditions” related to interactions with school buses. It maintained that its software has improved since the issue was first raised.

The probe adds to a string of mishaps by Waymo in recent months even as it aims to expand service across the U.S. this year. In December, its robotaxis froze and blocked other cars during a major power failure in San Francisco as it was confused by the lack of traffic lights.

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