The US government has launched an investigation into the safety of automaker Tesla's autonomous driving system after what may be the world's first known death involving "self-driving" technology.

A driver of a Tesla Model S car operating the system, which is called Autopilot, was killed in a collision with a truck two months ago, prompting the probe, which was disclosed on Thursday.

The investigation comes as Tesla and other automakers are gearing up to offer systems that allow vehicles to drive themselves under certain conditions across a wide range of vehicles over the next few years.

The National Highway Traffic Safety Administration (NHTSA) said it was now investigating 25,000 Model S sedans that were equipped with Autopilot.

The accident, according to a report from the Florida Highway Patrol, killed Joshua Brown on a clear, dry roadway on May 7 in the state of Florida.

The crash will add fire 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.

Sky too bright

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

The luxury electric car maker said in a blog 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 shares fell as much as 3 percent in after-hours trading, on news of the crash and investigation.

The company emphasised the unusual nature of the crash and said it was the first fatality in more than 130 million miles of use.

Tesla said in a statement on Thursday that customers were required to give "explicit acknowledgement" that they realise Autopilot is new technology still under development, otherwise the system would stay off.

"When drivers activate Autopilot, the acknowledgment box explains, among other things, that Autopilot 'is an assist feature that requires you to keep your hands on the steering wheel at all times,' and that 'you need to maintain control and responsibility for your vehicle' while using it."

The NHTSA said the crash called "for an examination of the design and performance of any driving aids in use at the time of the crash".

The agency said it had opened a preliminary investigation, which it said was the first step before it could seek to order a recall if it was to find the vehicles were unsafe.

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Source: AP