The National Highway Traffic Safety Administration has opened a preliminary investigation into a fatal highway crash involving a 2015 Tesla Model S that was operating in Autopilot mode at the time.

“This preliminary evaluation is being opened to examine the design and performance of any automated driving systems in use at the time of the crash,” NHTSA stated in a document posted on the agency’s website.

According to a Tesla blog post, the Tesla Model S was traveling on a divided highway with Autopilot engaged when a tractor-trailer drove across the highway perpendicular to the car.

“Neither Autopilot nor the driver noticed the white side of the tractor trailer against a brightly lit sky, so the brake was not applied,” Tesla said in the blog. “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.”

If the Model S had struck the front or rear of the trailer instead, even at high speed, the vehicle’s advanced crash safety system would have likely prevented serious injury, Tesla said.

The company also stressed that Tesla disables Autopilot by default. The driver must acknowledge that the system is new technology and still in a public beta phase before the feature can be enabled. The acknowledgment box explains that Autopilot is “an assist feature that requires you to keep your hands on the steering wheel at all times” and that “you need to maintain control and responsibility for your vehicle” while using Autopilot.

Additionally, each time Autopilot is activated, the car reminds the driver: “Always keep your hands on the wheel. Be prepared to take over at any time.”

The system frequently checks to make sure the driver’s hands remain on the wheel and provides visual and audible alerts when hands are off. The car gradually slows down if the driver’s hands remain off.

“Autopilot is getting better all the time, but it is not perfect and still requires the driver to remain alert,” Tesla said. “Nonetheless, when used in conjunction with driver oversight, the data is unequivocal that Autopilot reduces driver workload and results in a statistically significant improvement in safety when compared to purely manual driving."

Neither Tesla's blog post nor the NHTSA document identified the crash victim by name. However, the automaker called him “a friend to Tesla and the broader EV community.” The company extended its “deepest sympathies to his family and friends.”

A Washington Post report identified the driver as Joshua David Brown, 40, of Canton, Ohio. At the time of the May 7 crash he was driving on U.S. Route-27 in Williston, Fla., the newspaper reported.