Autopilot enables cars to steer, accelerate and brake within their lanes without driver intervention but Tesla says the feature requires “active driver supervision and do not make the vehicle autonomous.”

Last month, NHTSA opened an investigation into a February fatal crash of a 2014 Tesla Model S involving a fire truck in Contra Costa County, Calif. The local fire department said a Tesla struck one of its fire trucks and the Tesla driver was pronounced dead at the scene.

In June, NHTSA upgraded to an engineering analysis its defect probe into 830,000 Tesla vehicles with driver assistance system Autopilot that involves crashes with parked emergency vehicles including fire trucks. That step is necessary before the agency could demand a recall.

NHTSA is reviewing whether Tesla vehicles adequately ensure drivers are paying attention. Previously, the agency said evidence suggested drivers in many crashes under review had complied with Tesla’s alert strategy that seeks to compel driver attention, raising questions about its effectiveness.

Tesla did not respond to a request for comment.