Tesla Inc. is facing scrutiny from US regulators who have received 758 complaints of cars made by the company that suddenly brake at high speeds, more than double the number of reported incidents earlier this year.

Complaints rose from 354 in February, prompting the National Highway Traffic Safety Administration to ask Tesla for a response by June 20, according to a regulatory filing on Friday. The agency opened a probe within the month into the “phantom braking” phenomenon, covering an estimated 416,000 Tesla Model 3 and Model Y vehicles.

Although no crashes or injuries from phantom braking have been reported, Tesla shares fell 6.2% to $727.06 as of 10:00 a.m. in New York.

“That is a huge number of complaints in a short time, and indicates that NHTSA’s probe should be stepping up,” said Michael Brooks, acting executive director and chief counsel of the Center for Auto Safety. “We have hundreds of owners per month reporting false activation of their emergency braking systems, and untold numbers of others not reporting the issue to NHTSA.” 

The phantom braking probe was launched in February, two weeks after NHTSA announced it was reviewing complaints about Tesla’s forward-collision avoidance system.

NHTSA also has other investigations into Tesla’s technology. In August 2021, it opened a probe into a possible defect of its partially automated driver-assistance feature, looking into how the system handles crash scenes following a dozen collisions with first responders and other vehicles. 

In December, NHTSA also launched an evaluation of the company allowing car occupants to play video games on front-center touch screens. The carmaker told the agency it would work on a software update to lock the feature when vehicles are in motion.