In the first-ever fatality linked to a driverless car, a Tesla Model S with the Autopilot system activated was involved in a fatal crash on May 7 in Williston, Fla. This is the first known fatality in a Tesla where Autopilot was active.

The company revealed the crash in a blog post posted today and says it informed the National Highway Transportation Safety Administration (NHTSA) of the incident, which it is now investigating. NHTSA is opening a preliminary evaluation into the performance of Autopilot.

The model S has software that is considered “semi-autonomous,” because a driver is still required to be behind the wheel, but the car can change lanes, brake, steer, accelerate, decelerate and avoid obstacles on its own when on autopilot.

The accident, which occurred on a divided highway in central Florida, involved the Tesla and a tractor trailer. The vehicle had Autopilot engaged when the tractor trailer drove across the highway perpendicular to the Model S, said Tesla.

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 has already absolved itself of liability by saying the Autopilot feature is still in beta phase, and drivers are still ultimately responsible for controlling the vehicles.

[It is important to note that Tesla disables Autopilot by default and requires explicit acknowledgement that the system is new technology and still in a public beta phase before it can be enabled. When drivers activate Autopilot, the acknowledgment box explains, among other things, 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 it. Additionally, every time that Autopilot is engaged, the car reminds the driver to “Always keep your hands on the wheel. Be prepared to take over at any time.” The system also makes frequent checks to ensure that the driver’s hands remain on the wheel and provides visual and audible alerts if hands-on is not detected. It then gradually slows down the car until hands-on is detected again.]

The investigation by NHTSA will raise questions regarding the safety of self-driving vehicles on the roads. This comes at a time when federal regulators and states are beginning to craft policies on autonomous vehicles.