Washington, DC CNN  — 

The National Highway Traffic Safety Administration is investigating reports of Teslas braking unexpectedly while using Autopilot.

The braking occurs “without warning, at random, and often repeatedly in a single drive,” according to NHTSA. It estimates that 416,000 vehicles may be affected. The agency did not say whether it was aware of any collisions, injuries, or fatalities associated with the issue.

The agency’s office of defects investigation has received 354 complaints alleging unexpected braking on 2021-22 Tesla Model 3s and Tesla Model Ys in the past nine months. The alleged braking occurs while using Autopilot, Tesla’s suite of driver-assist features, which allow the vehicle to brake and steer automatically. Tesla owners sometimes refer to the unexpected brake activation as “phantom braking.”

NHTSA’s investigation focuses on Teslas produced since the automaker removed radar from most of its vehicles, choosing to rely exclusively on cameras for its driver-assist features. NHTSA is not investigating the Model X and Model S, which Tesla did not immediately remove radar from.

One person filing a complaint with NHTSA alleged that their new Tesla, which lacks radar, has more phantom braking issues than their previous Tesla, which had radar.

“The new Model Y with vision only is unreliable and dangerous,” they wrote. “The vehicle will randomly brake with no traffic in broad daylight.”

Another person who filed a complaint alleged that the problem occurred more than 20 times while on a road trip from California to Colorado.

“I believe this problem is extremely dangerous and have not turned on the autopilot features since October,” the person wrote.

Tesla did not respond to a request for comment.

The agency’s investigation is technically a “preliminary evaluation,” which can be a precursor to an automaker issuing a recall. NHTSA had opened a preliminary evaluation into Tesla’s Boombox feature last fall before the automaker issued a safety recall last month. (The feature was recalled because it could negatively impact the pedestrian warning system and make a crash more likely.)

The preliminary evaluation into the unexpected braking will “determine the scope and severity of the potential problem and to fully assess the potential safety-related issues,” according to NHTSA.

Teslas has faced a string of recent recalls, including issues with a seat belt alert and its “full self-driving” software.

Electric vehicles like Teslas can often be fixed through over-the-air software updates. Such fixes can be made quickly and aren’t as inconvenient or financially burdensome for drivers and automakers as traditional recalls.

CNN’s Gregory Wallace contributed to this report.