The National Transportation Safety Board (NTSB) has opened an investigation into Waymo after its robotaxis have been spotted illegally passing stopped school buses numerous times in at least two states.

The NTSB is specifically focusing on the more than 20 incidents that have occurred in Austin, Texas, it said in a post on X on Friday.

“Investigators will travel to Austin to gather information on a series of incidents in which the automated vehicles failed to stop for loading or unloading students,” the NTSB said in a statement to TechCrunch. A preliminary report is expected within 30 days, and the safety board will publish a more detailed final report in 12 to 24 months.

It’s the first time Waymo has been investigated by the NTSB, but it’s the second investigation launched into Waymo over its school bus problem. The National Highway Traffic Safety Administration’s (NHTSA) Office of Defects Investigation opened a similar probe in October.

Waymo also issued a software recall last year to address the problem. But previous software updates have not been enough to stamp it out, and in Austin, Texas — where the bulk of incidents have been captured on camera — the school district has asked the company to suspend operations during pickup and drop-off times.

The new investigation comes as Waymo is in the middle of a rapid expansion around the United States. Just this week, the company started offering a robotaxi service in Miami, adding to its operations in Atlanta, Austin, Los Angeles, Phoenix, and the San Francisco Bay Area.

“We safely navigate thousands of school bus encounters weekly across the United States, and the Waymo Driver is continuously improving. There have been no collisions in the events in question, and we are confident that our safety performance around school buses is superior to human drivers,” Mauricio Peña, Waymo’s chief safety officer, said in a statement to TechCrunch. “We see this as an opportunity to provide the NTSB with transparent insights into our safety-first approach.”

The NTSB is different from the NHTSA in that it is not a federal regulatory agency. It cannot issue fines or penalties. Rather, the safety board usually performs deep investigations to identify root causes of problems in the transportation world. When an investigation is complete, the board often holds hearings and issues non-binding recommendations.

The first notable incident where a Waymo vehicle passed a stopped school bus happened last September in Atlanta. The Waymo pulled out of a driveway and crossed perpendicularly in front of the school bus from the bus’ right side. The robotaxi then turned left and proceeded down the street while kids were getting off the bus.

Waymo said at the time that the vehicle was unable to see the stop sign or flashing lights, and has since said that it addressed this particular scenario with a software update.

But as Waymo patched the specific scenario it encountered in Atlanta, some of the company’s vehicles were caught passing stopped school buses in Austin, Texas. Local news outlet KXAN published videos it received from cameras mounted on school buses that showed Waymo vehicles making illegal maneuvers on multiple occasions.

“We continue to engage productively with the Austin Independent School District and applaud their reported success in reducing human-driven violations around school buses from 10,000+ a year,” Peña said.