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.

According to a filing, Waymo issued a fix for the software issue in mid-November, and an official voluntary recall a month later. But the government says that the majority of school bus issues occurred after that date.

Earlier this year, a report from the National Transportation Safety Board, the US’ independent safety agency, found that another school bus passing incident occurred in mid-January, several weeks after the supposed software fix. The NTSB is also investigating the school bus passing issue.

In a written statement, Waymo spokesperson Dana Gallagher said the company has “made significant improvements to vehicle behavior around school buses,” and that it has not been cited for passing school buses since last March—months after the mid-December software recall. She wrote that Waymo looks “forward to continuing our engagement on recall processes for AV software that continues to learn and improve.”

Late last year, WIRED has reported, the Austin school district even helped set up a half-day data collection event for Waymo’s vehicles, coordinating with the self-driving vehicle developer to bring school buses and stop-arm signals to an empty parking lot so that the company could collect information to help train their vehicles to avoid the illegal street maneuver. The data collection event occurred after Waymo had issued its software fix and voluntary recall. Still, the incidents continued.

In years past, recall queries like this one have led to civil penalties, some of them costing automakers and software developers tens of millions of dollars.