New York: Ths US authorities have launched an investigation after the tragic death of a man driving electric automobile-maker Tesla's autonomous car in a crash - the first known fatal accident involving a self-driving vehicle being driven in autopilot mode. Joshua Brown, 40, of Ohio was killed when his Tesla Model S electric sedan crashed into a tractor-trailer while reportedly in autopilot mode. Preliminary investigation indicated that the crash took place when the tractor-trailer made a turn in front of the Tesla and the car failed to apply the brakes, the US National Highway Traffic Safety Administration (NHTSA) said in a statement. NHTSA added that nearly 25,000 Tesla Model S sedans that are equipped with the autopilot system have been put under investigation. "What we know is that the vehicle was on a divided highway with autopilot engaged when a tractor trailer drove across the highway perpendicular to the Model S. 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. However, the company also said that "this is the first known fatality in just over 130 million miles where autopilot was activated. Among all vehicles in the US, there is a fatality every 94 million miles. Worldwide, there is a fatality approximately every 60 million miles". According to the New York Times, the tragic death of Brown - a Navy veteran who owned a technology consulting firm - on May 7 this year in Florida comes at a time when regulators are in the early stages of setting guidelines for autonomous vehicles. The death is a blow to Tesla at a time when the company is pushing to expand its product lineup from expensive electric vehicles to more mainstream models, NYT reported. "It is important to note that Tesla disables autopilot by default and requires explicit acknowledgement that the system is new technology and still in a public beta phase before it can be enabled," Tesla said in its its defence. "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 automaker added. Companies are increasingly investing in self-driving technology that rely heavily on sophisticated computer software, sensors, cameras and radar. Google recently announced plans to adapt 100 Chrysler minivans for autonomous driving, while General Motors earlier this year acquired the software firm Cruise Automation to accelerate its own self-driving applications. Although NHTSA said the opening of a probe did not mean there was a defect in the vehicle being examined, Brown's accident has only strengthened scepticism about the selfdriving technology and may make the approval process for such cars more stringent.