Tesla Motors Inc. said in a blog post Thursday it was being investigated by the National Highway Traffic Safety Administration for a fatal crash involving the company’s Model S sedan. The NHTSA opened “a preliminary investigation into the performance of the autopilot” feature of the car following the accident in May.

A driver was killed in Florida in May while using the self-drive feature in the 2015 Model S. According to Tesla, “the vehicle was on a divided highway with autopilot engaged when a tractor trailer drove across the highway perpendicular to the Model S. Neither autopilot nor the driver noticed the white side of the tractor trailer against a brightly lit sky, so the brake was not applied. The high ride height of the trailer combined with its positioning across the road and the extremely rare circumstances of the impact caused the Model S to pass under the trailer, with the bottom of the trailer impacting the windshield of the Model S.”

The company did not identify the customer but said: “He was a friend to Tesla and the broader EV community, a person who spent his life focused on innovation and the promise of technology and who believed strongly in Tesla’s mission.”

Various news outlets, citing the Florida Highway Patrol, identified the deceased as Joshua Brown. In April, Brown posted a video on YouTube that showed the car swerving to avoid a truck. He also called the Model S “hands down the best car I have ever owned.” His death is the first known fatality linked to using the self-drive feature of the Model S.

Tesla’s shares had gained 0.99 percent on Nasdaq during Thursday trade but fell 3.95 percent during after-market hours.