The federal government’s main auto-safety regulator has opened a preliminary investigation into sudden braking by Tesla cars equipped with an advanced driver-assistance system that the company calls Autopilot.

The National Highway Traffic Safety Administration said in a document posted online this week that it was acting in response to 354 consumer complaints from the last nine months about “phantom braking” — when Teslas brake unexpectedly when no hazards are in the road.

The investigation focuses on Tesla Model 3 compact sedans and Model Y hatchbacks that were made in 2021 and 2022 and sold in the United States. The agency said that covers about 416,000 cars. The preliminary inquiry is intended to determine the scope and severity of the problem.

The complaints claim that the vehicle “unexpectedly applies its brakes while driving at highway speeds,” the safety agency wrote in a summary of the investigation. “Complainants report that the rapid deceleration can occur without warning, at random, and often repeatedly.”

Tesla did not respond to a request for comment.

The investigation is the latest safety concern that Tesla has been confronted with. Separately this year, Tesla recalled 54,000 cars equipped with its Full Self-Driving software to disable a feature that in certain conditions let the vehicles roll slowly through intersections without stopping. Failing to stop at a stop sign can increase the risk of a crash, the safety agency told Tesla in a letter.

The agency also opened a formal investigation last year into Autopilot and how it identifies objects and other vehicles on the road. That inquiry was prompted by 11 incidents in which Teslas operating in Autopilot failed to stop for and crashed into police cars, fire trucks and other emergency vehicles that had flashing lights turned on.

In the last few months, Tesla recalled 12,000 cars to correct a braking problem and recalled 458,000 others for two separate mechanical defects. It also agreed to turn off a feature that allowed drivers or front passengers to play video games on the dashboard touch-screen while car were in motion. The problem had been highlighted in The New York Times, and the safety agency pressed the company to address it.