By David Shepardson

WASHINGTON (Reuters) -The National Highway Traffic Safety Administration (NHTSA) said Thursday it is opening a formal investigation into 416,000 Tesla vehicles over reports of unexpected brake activation tied to its driver assistance system Autopilot.

The preliminary evaluation covers 2021-2022 Tesla Model 3 and Model Y vehicles in the United States after the agency received 354 complaints about the issue over the past nine months. NHTSA said the vehicles under review have a advanced driver assistance system that Tesla calls Autopilot that allows them to brake and steer automatically within its lanes.

NHTSA said: “Complainants report that the rapid deceleration can occur without warning, at random, and often repeatedly in a single drive cycle.”

Earlier this month, NHTSA confirmed it was reviewing consumer complaints that Tesla vehicles were activating the brakes unnecessarily. A preliminary evaluation is the first phase before NHTSA could issue a formal recall demand.

In May, Tesla chief executive Elon Musk said dropping a radar sensor from its partially automated driving system would address the so-called “phantom braking” issue, which some Tesla drivers have long complained about. Tesla, which disbanded its media relations department, did not respond to a request for comment.

Tesla has come under increasingly scrutiny from NHTSA, which is investigating several issues and the electric vehicle manufacturer has issued 10 recalls since October, including some under pressure from the agency.

In November, Tesla recalled nearly 12,000 U.S. vehicles sold since 2017 because a communication error may cause a false forward-collision warning or unexpected activation of the emergency brakes.

The recall was prompted after a software update on Oct. 23 to vehicles in its limited early access version 10.3 Full-Self Driving (FSD) (Beta) population.

FSD is an advanced driver assistance system that handles some driving tasks but Tesla and NHTSA say it does not make vehicles autonomous.

The recall came after NHTSA in October asked Tesla why it had not issued a recall to address software updates made to its Autopilot driver-assistance system to improve the vehicles’ ability to detect emergency vehicles.

NHTSA in August opened a formal safety probe into Tesla’s Autopilot system in 765,000 U.S. vehicles after a series of crashes involving Tesla models and emergency vehicles.

(Reporting by David Shepardson; Editing by Toby Chopra and David Evans)