PALO ALTO (KCBS) — Tesla says the National Highway Traffic Safety Administration is investigating the performance of the autopilot system in a Model S sedan during a fatal crash.

Tesla released a blog entry Thursday on the investigation, saying that this is the first known fatality while the autopilot system was in use.

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The Model S was on a divided highway in Levy County, Florida, with the autopilot system engaged when a tractor trailer drove across the highway ahead of the Tesla.

Tesla stated that neither the driver nor the autopilot system acknowledged the white side of the trailer against a “brightly lit sky,” and the brakes were not applied before the sedan crashed into the underside of the trailer.

Tesla said “extremely rare circumstances” combined with the trailer’s height, and its position across the road caused the car to pass underneath the trailer.

MORE: Read Tesla’s Full Blog Post Here

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The Levy County Journal indentified the Tesla driver as 45-year-old Joshua Brown of Ohio. The Journal report said the roof of Brown’s Tesla was torn off by the force of the collision. The truck driver, Frank Baressi, 62, Tampa was not injured in the crash, according to the report.

The Journal said charges were pending in the case, but it was not known whether alcohol or drugs were a factor.

Tesla emphasized that the investigation is only a preliminary evaluation of whether the system worked as it was intended.  Tesla disables the autopilot system by default, and the system has safety features to ensure drivers keep their hands on the wheel while the system is engaged.

The identity of the driver, and the location of the crash were unknown.

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