The National Transportation Safety Board announced Friday it has opened an investigation into Waymo robotaxis following a series of incidents in Austin, Texas, involving school buses filled with children. 

NTSB investigators will travel to Austin to gather information on a series of incidents in which the automated vehicles failed to properly yield for buses with the Austin Independent School District while they were loading or unloading students with their lights flashing and stop signs activated.

Waymo cars have received at least two dozen such violations in Austin so far.

“We safely navigate thousands of school bus encounters weekly across the United States, and the Waymo Driver is continuously improving,” Mauricio Peña, Waymo chief safety officer, said in a statement provided to CBS News on Friday. “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. 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.”

Waymo previously said it launched a software update in November to address the issue, but it has received at least four violations since then.

The school district that same month asked Waymo to halt operations when its school buses are operating. At the time, a school district spokesperson told CBS News that Waymo had “refused to cease operations” during the hours the school district had requested.

“We have seen material improvement in our performance since our software update,” a Waymo spokesperson said earlier this month.

Waymo said it met with the school district last month to better understand the issue.

The NTSB’s probe could take 12 to 14 months to complete, but a preliminary report will be released within 30 days.

“School bus safety and the safe operation of automated vehicles have been a focus of previous NTSB investigations,” the NTSB said in its statement announcing the probe.

Last month, the National Highway Traffic Safety Administration said in a letter to Waymo that it was expanding its own investigation into Waymo in response to the Austin school bus incidents.