Tesla stock fell 3.2% to $380.30.
The increased regulatory scrutiny comes as Tesla is trying to convince investors that the future of the company lies less in selling cars as sales drop and more in making its self-driving software ubiquitous. Musk has said he will soon turn millions of Tesla cars already on the road into taxis that their owners can rent out when they are not using them.
As part of that transition, Musk said Tesla will roll out its robotaxi service with no one behind the wheel in several U.S. cities this year. It is also planning to launch production of its no-wheel-no-pedal Cybercab to sell to customers next month.
Tesla did not immediately respond to a request for comment.
Unlike other autonomous vehicles, Tesla vehicles rely solely on cameras to spot problems on the road. Others supplement cameras with light radar or lidar, a more expensive method that Musk has dismissed as unnecessary.
The NHTSA probe into crashes when there is sun glare or dust or too much fog will now move to an “engineering analysis,” a more serious level of scrutiny.
Tesla had called its driver assistance software Full Self-Driving, or FSD, a name that auto experts and regulators have said is misleading because drivers must always keep their eyes on the road and be ready to take over at any time. The company has since changed the name to Full Self-Driving (Supervised).
Of the nine crashes under review, Tesla has told regulators that three would not have occurred with new over-the-air FSD upgrades.
Tesla is facing several regulatory other probes including one over its FSD-equipped cars running red lights and another on door handles that reportedly failed to work in crashes, trapping passengers inside.