Tesla has falsely suggested in social media posts that its cars are autonomous, potentially misleading and endangering drivers, according to a warning from the National Highway Traffic Safety Administration.

“Tesla’s X (Twitter) account has re-posted or endorsed postings that exhibit disengaged driver behavior,” Gregory Magno, the head of the NHTSA, wrote to Tesla’s board in an email in May that was made public on Friday. “Tesla’s postings conflict with its stated messaging that the driver is to maintain continued control over the dynamic driving task.”

Magno warned that viewers of Tesla’s online communications, including posts on X, may expect a car with the company’s Full Self-Driving Supervised, or FSD, system to have the capabilities of a robotaxi even though they require human supervision, meaning a driver ready to steer or brake at any time.

Tesla has until Dec. 18 to respond to NHTSA’s letter, which also included requests for extensive information about the company’s cars and FSD-related technology. A failure to respond could result in a maximum of $135.8 million in penalties. The company can also ask for an extension.

The NHTSA is investigating possible safety defects in Tesla’s FSD option, a partially automated driving system. The latest probe followed a series of Tesla collisions that involved the use of FSD within 30 seconds of impact. In one instance, a Tesla driver using FSD fatally struck a pedestrian.

Magno referred to an array of posts shared by Tesla on X, the social network owned by Tesla CEO Elon Musk. One post showed a driver using FSD to go to a hospital while enduring a possible heart attack. Another showed a driver who said he used FSD for a 50-minute ride home from a sporting event.

In the posts, Tesla suggested there are “advantages to using FSD while inebriated” or while fatigued, Magno wrote. NHTSA asked Tesla to make its online communications, “consistent” with user instructions, and “appropriate to the level of capability deployed to the public roads.”

Musk, who emerged as the principal backer of President-elect Donald Trump’s campaign, aims to cut federal spending and regulations when the new administration begins. He also seeks to create a federal motor vehicle safety standard for autonomous vehicles.

Tesla shares have climbed 28% in the three trading days since Tuesday’s election, surpassing a market cap of $1 trillion on Friday.

Tesla didn’t immediately respond to a request for comment. Read the NHTSA letter to Tesla here.

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