A former Navy SEAL named Joshua Brown was killed when his self-driving Tesla Model S drove under a tractor-trailer, sheering the roof off his car. According to the National Highway Traffic Safety Administration, this is the first fatal crash involving a self-driving vehicle.

Tesla published a statement about the crash on its website which describes the circumstances of the crash. It reads in part:

We learned yesterday evening that NHTSA is opening a preliminary evaluation into the performance of Autopilot during a recent fatal crash that occurred in a Model S. This is the first known fatality in just over 130 million miles where Autopilot was activated. Among all vehicles in the US, there is a fatality every 94 million miles…

Following our standard practice, Tesla informed NHTSA about the incident immediately after it occurred. What we know is that 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 statement goes on to say, “Autopilot is getting better all the time, but it is not perfect and still requires the driver to remain alert.”

There is some question about how the driver missed seeing the tractor trailer. The Dallas Morning News reports that Frank Baressi, the driver of the truck, claims Brown was watching a movie on the dashboard at the time of the crash. Baressi didn’t actually see the movie but claims to have heard it playing after the crash. However, Tesla says it is not possible to watch a movie on the dashboard touchscreen of the Model S.

Brown was a Navy seal for 11 years until 2008. He owned a technology company and was very enthusiastic about his Model S which he nicknamed “Tessy.” He posted videos on You Tube specifically praising the car’s autopilot feature. In this clip from April, Brown claims the autopilot prevented a crash by avoiding a truck that swerved into his lane:

At the start of this video you can hear that Brown is listening to what sounds like an audio book just before the incident and in the description beneath the video he writes, “Note 2: In case you’re curious, I’m listening to an audiobook in the background. It’s a Malcolm Gladwell book (excellent book).” Perhaps the sound the truck driver interpreted as Brown watching a movie (which, again, Tesla says is not possible) was actually an audiobook playing in the car.

Tesla founder Elon Musk tweeted his condolences and also, in response to a pointed question, explained why the cars “radar” didn’t recognize the truck and apply the brakes:

Our condolences for the tragic loss https://t.co/zI2100zEGL

— Elon Musk (@elonmusk) June 30, 2016

Radar tunes out what looks like an overhead road sign to avoid false braking events

— Elon Musk (@elonmusk) June 30, 2016