Electric car maker Tesla is under investigation in the US after the first fatal accident involving its autopilot function.
Tesla said in a statement that a Model S was 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," according to Tesla.
The company said the height of the trailer combined with its positioning across the road caused the Model S to pass under the trailer, with the bottom of the trailer impacting the windshield.
The accident occurred on May 7 in Florida, and Tesla says the National Highway Traffic Safety Administration Authority (NHTSA) was notified immediately.
Tesla claims it's the first known death in more than 209 million kilometres where autopilot was activated. Among all vehicles in the US, there is a fatality every 151 million kilometres.
In a statement, the NHTSA said the beginning of a preliminary investigation "should not be construed as a finding that" the agency "believes there is either a presence or absence of a defect in the subject vehicles".
Tesla says it disables autopilot by default and requires drivers to acknowledge that the system is new technology. It also says drivers are told to keep their hands on the wheel at all times and maintain control and responsibility.
"The customer who died in this crash had a loving family and we are beyond saddened by their loss," Tesla said in a statement.
"He was a friend to Tesla and the broader [electric vehicle] community, a person who spent his life focused on innovation and the promise of technology and who believed strongly in Tesla's mission. We would like to extend our deepest sympathies to his family and friends."