WASHINGTON, D.C. A U.S. agency says it has requested more information from Waymo after Texas officials reported that the company's self-driving cars illegally passed school buses 19 times since the start of the school year.

The National Highway Traffic Safety Administration (NHTSA) opened an investigation in October after a Waymo vehicle in Georgia failed to stay stopped for a school bus with flashing red lights and a deployed stop sign.

In a November 20 letter, the Austin Independent School District said five more incidents occurred this month, even after Waymo said it had updated its software to fix the problem. The district asked Waymo to stop operating near schools during pick-up and drop-off times until it could guarantee the vehicles would comply with the law.

"We cannot allow Waymo to continue endangering our students while it attempts to implement a fix," the district's lawyer wrote, pointing to a case where a Waymo car passed a stopped bus moments after a child crossed in front of it.

As a result, NHTSA asked Waymo on November 24 whether it would comply with the request to halt operations near schools and whether the software update was sufficient. The agency also requested whether Waymo plans to issue a recall.

The school district told Reuters that Waymo has refused to stop operations around schools and reported another incident on December 1.