Federal automotive safety regulators opened an investigation into a Tesla recall of all models equipped with the Autopilot driver-assistance system to determine if installed software updates were enough to address the issue.

Tesla recalled some two million vehicles in December, saying Autopilot controls might be “insufficient to prevent driver misuse.”

The National Highway Traffic Safety Administration says Tesla indicated it would make a “multipart remedy aimed at improving system and engagement controls and reducing mode confusion.”

The transportation department agency two years earlier opened an investigation into Autopilot that has pinpointed at least 13 fatal accidents, plus “many more” resulting in serious injuries in which “foreseeable driver misuse of the system played an apparent role.”

After Tesla’s recall last year, NHTSA said post-recall accidents and some agency testing prompted concerns, along with Tesla saying that part of the recall remedy requires driver opt-in and allows for the driver to reverse the remedy. The agency said Tesla has made non-recall updates that “appear related” to NHTSA’s concerns.

“This investigation will consider why these updates were not a part of the recall or otherwise determined to remedy a defect that poses an unreasonable safety risk,” NHTSA said in its investigation summary.

LEARN MORE: Cybertrucks Recalled Over Crash Risk