The US government is examining whether Waymo’s recall of software involved with a series of school bus passing incidents last fall actually fixed the safety issue.

The formal investigation by the National Highway Traffic Safety Administration (NHTSA) will determine whether the company’s supposed software fix succeeded in preventing its robotaxis from improperly passing stopped school buses with red lights on and their stop arms extended—indicators that the buses were picking up and dropping off students. It is illegal to pass these stopped school buses in all 50 states.