Tesla Inc. was admonished by a US agency over how the carmaker was promoting its driver-assistance technology on social media months before the regulator opened a defect investigation of the system.

In a May 15 email sent to Tesla representatives, a National Highway Traffic Safety Administration official flagged seven social media posts the company had shared that gave the agency pause. Each post on X, the service that Tesla Chief Executive Officer Elon Musk owns, showcased disengaged drivers using the system Tesla has marketed as Full Self-Driving, or FSD.

"We believe that Tesla's postings conflict with its stated messaging that the driver is to maintain continued control over the dynamic driving task," Gregory Magno, a division chief within NHTSA's Office of Defects Investigation, wrote in the letter posted on the agency's website Friday. "These postings may encourage viewers to see FSD-Supervised as a Chauffer or 'Robotaxi' rather than a partial automation / driver-assist system that requires persistent attention and intermittent intervention by the driver."

The email offers a sense of how closely NHTSA was monitoring Tesla and FSD in the months before the regulator decided to investigate the system. Magno notes that the agency had asked the company to brief NHTSA's technical staff about how the company had begun offering free trials of FSD. He wrote that Tesla had obliged and emphasized ways it had communicated to drivers that their vehicle wasn't autonomous.

Before and after providing that briefing to NHTSA, Tesla published reposts on X that Magno suggested were problematic. The examples he cited included:

-A Tesla sport utility vehicle owner who wrote that he'd used FSD to get to the hospital while enduring a mild heart attack

-A video of a Tesla owner using FSD for a drive from the parking garage of a sporting event that led third-party commenters to write they "can foresee more drinkers getting home safely"

-A video of a Tesla driver admitting to having been inattentive to their surroundings

"While Tesla has the discretion to communicate with the public as it sees fit, we note that these posts show lost opportunities to temper enthusiasm for a new product with cautions on its proper use with the points that Tesla has made to us," Magno wrote.

Tesla didn't respond to an emailed request for comment on the email.

NHTSA opened its investigation of FSD in October, citing four collisions involving cars that had the system engaged. In one of the crashes, a Tesla Model Y SUV fatally struck a pedestrian.

The agency is assessing whether the system has the ability to detect and appropriately respond to fog and other reduced visibility conditions. Magno's email was included in an information request NHTSA sent to Tesla dated Nov. 5, the day of the US presidential election.

The regulator posted the filing on its website Friday and has given the company until Dec. 18 to address a series of questions regarding FSD.