A fatal accident in which the driver of a Tesla Motors Model S car operating in Autopilot mode was killed in a collision with a truck has prompted an investigation by federal highway safety regulators. The investigation of the first known fatality to involve a Model S operating on Autopilot comes as Tesla and other automakers are gearing up to offer systems that allow vehicles to pilot themselves under certain conditions across a wide range of vehicles.

The National Highway Traffic Safety Administration said it is investigating 25,000 Model S sedans that are equipped with the Autopilot system.

The accident will add fuel to a debate within the auto industry and in legal circles over the safety of systems that take partial control of steering and braking from drivers.

The NHTSA said preliminary reports indicate the crash occurred when a tractor-trailer made a left turn in front of the Tesla at an intersection.

Tesla said in a blogpost on Thursday that "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 said in a statement on Thursday that customers are required to give "explicit acknowledgement" that they realize Autopilot is new technology still under development, otherwise the system will remain off.

The NHTSA said the crash "calls for an examination of the design and performance of any driving aids in use at the time of the crash." The agency said it has opened a preliminary investigation that is the first step before it could seek to order a recall if it finds the vehicles were unsafe.