According to Reuters, the incident took place on May 7 near Williston, Florida, and is currently being investigated by the US National Highway Traffic Safety Administration (NHTSA).
The agency told The New York Times: "Preliminary reports indicate the vehicle crash occurred when a tractor-trailer made a left turn in front of the Tesla at an intersection on a non-controlled access highway".
UPDATE (July 2, 2016): Frank Baressi has told the Associated Press that he was driving the truck that the Tesla crashed into.
According to Baressi, the Model S "went so fast through my trailer I didn't see him". Baressi claims that the driver of the Tesla was watching Harry Potter on a TV screen, although he was only able to hear the movie playing.
Tesla confirmed to the news agency that its touchscreen infotainment system doesn't allow videos to play. The movie is reportedly not cited in the original police report.
ORIGINAL ARTICLE (continued): In a blog post, Tesla Motors said: "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."
Tesla says that this is the first known fatality in a Model S where the Autopilot system was active. So far Model S drivers have used Autopilot for a total of 209 million kilometres.
According to the electric car maker, in the US there's a death for every 151 million kilometres driven. Globally, there's a fatality every 97 million kilometres driven.
The Autopilot system present on Tesla Model S sedans and Model X SUVs requires drivers to keep their hands on the steering wheel. If the driver takes their hands off the wheel, the system will eventually notice, and give out visual and audio alerts before gradually bringing the car to a halt.
The Californian automaker insists that "the data is unequivocal that Autopilot reduces driver workload and results in a statistically significant improvement in safety when compared to purely manual driving".