The National Highway Traffic Safety Administration has opened an investigation into Waymo following reports that the company’s robotaxis illegally drove past stopped school buses in Austin, Texas.

In a letter sent to Waymo on Wednesday, the federal regulator’s Office of Defects Investigation asked for detailed information on Waymo’s fifth-generation self-driving system and expressed concern that the Alphabet-owned company’s vehicles were showing unexpected or illegal behavior around school buses.

The letter comes after the watchdog opened an October investigation into the firm following an incident involving a stationary school bus in Georgia. Video footage showed one of Waymo’s autonomous vehicles driving past a stopped school bus with its red lights flashing and stop arms deployed. In response, Waymo said safety is its top priority and that it had already issued software updates to its fleet to resolve the issue.

However, the problem appears to have persisted. In a letter dated November 20, the Austin Independent School District said it had evidence of 19 separate incidents in which Waymo vehicles had been recorded driving past stopped school buses since the start of the school year. At least five of these happened after the company said it issued an update to fix the problem.

The school district asked Waymo to cease self-driving operations around school pick-up and drop-off times. The district told Reuters the company had refused to halt operations around schools and claimed another incident involving an actively loading school bus occurred on December 1. In an email dated November 24, the NHTSA explicitly asked Waymo to “let us know if you have ceased operations during these times as requested or plan to cease,” and to confirm whether a software fix had been implemented.

It’s not the first time Waymo has come under regulatory scrutiny for its self-driving cars. For example, in 2024, the federal regulator launched a probe following reports of erratic driving that potentially broke the law. It comes as the company expands into new markets, with hopes to launch in over 20 cities in the coming years.

A Waymo spokesperson was not immediately available to comment on the probe.