The NHTSA cautioned that it was too early in the investigation to determine whether a defect existed or played a part in the crash.
Tesla Motors Inc said on Thursday a driver was killed in a collision with a truck while using its "Autopilot" self-driving mechanism on its Model S electric car, prompting a United States (US) federal highway safety investigation.
The National Highway Traffic Safety Administration (NHTSA) said that it has opened a "preliminary evaluation" into the performance of the "Autopilot" after the electric car company notified the agency of a fatal crash in Florida in May.
A report from the Florida Highway Patrol said that 40-year-old Joshua Brown was killed on a clear, dry roadway on May 7 in Williston, Florida.
The investigation of the first known fatality to involve a Model S operating on autopilot comes amid growing interest in self-driving cars following tests over the past few years by Google and research by several major automakers.
Investments are also being made in a wide range of vehicles including autonomous trucks and small buses.
Around 25,000 Model S sedans that are equipped with the autopilot system will be probed by NHTSA.
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.
Luxury electric car maker 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 shares fell as much as 3 percent, or $6.28, in after-hours trading, on the news of the fatal crash and the investigation.
The company emphasized the unusual nature of the crash and said it was the first fatality in more than 130 million miles of use.
Tesla Model S sedans start at about $66,000.
Tesla said that customers are required to give "explicit acknowledgement" that they realize Autopilot is new technology still under development, otherwise the system will remain 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.'"
The statement added that 'you need to maintain control and responsibility for your vehicle' while using it."
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.
It also cautioned that it was too early in the investigation to determine whether a defect existed or played a part in the crash.
Autopilot in beta mode
A report by the Florida Highway Patrol reviewed by Reuters states that the Model S operated by Brown went underneath the trailer of a truck that had turned left in front of the car.
The Tesla's windshield hit the bottom of the trailer as it passed underneath, and the car kept going, leaving the road, striking a fence, crossing a field, passing through another fence and finally hitting a utility pole about 100 feet south of the road, the report said.
Tesla said "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."
When Autopilot launched in October, Musk cautioned that the hotly anticipated function was in beta mode, or a test phase of development, with full "hands-off" driving not recommended.
A host of subsequent videos posted by Tesla drivers on YouTube showed near-misses on the road with Autopilot, prompting Musk to say he might curb the function to minimize the possibility of people doing "crazy things."
In January, Tesla updated the Autopilot driving systems in Model S sedans to put new limits on its hands-free operation, which has been both praised for its innovation while criticized for having been launched too early.
The function was restricted on residential roads or roads without a center divider, meaning the car cannot drive faster than the speed limit maximum plus five miles (8 km) per hour.
Tesla said on Thursday that "Autopilot is getting better all the time, but it is not perfect and still requires the driver to remain alert.
Nonetheless, when used in conjunction with driver oversight, the data is unequivocal that Autopilot reduces driver workload and results in a statistically significant improvement in safety when compared to purely manual driving."
The police report identified Brown as being from Canton, Ohio. An obituary available at legacy.com described Brown as the owner of Nexu Innovations in Stow, Ohio, and said that he died in "a tragic motor vehicle accident Saturday, May 7, 2016."
A biography on the Nexu website describes Brown as a former Navy SEAL.
Efforts to reach relatives listed in the obituary were not successful. Tesla declined to discuss the identity of the victim.
Tesla's blog post described the customer who died as a "friend to Tesla and the broader EV community, a person who spent his life focused on innovation and the promise of technology and who believed strongly in Tesla's mission."
Tesla Chairman Elon Musk, in a posting on his Twitter account late on Thursday, wrote: "Our condolences for the tragic loss."
A YouTube account belonging to a Joshua Brown whose personal details, including the company where he worked, match those of the accident victim, includes a video posted on April 5 titled "Autopilot Saves Model S."
In the video, a bucket truck, the type used by people working on utility poles, cuts off a Model S.
The written description of the video states, "The truck tried to get to the exit ramp on the right and never saw my Tesla. I actually wasn't watching that direction and Tessy (the name of my car) was on duty with autopilot engaged.
I became aware of the danger when Tessy alerted me with the "immediately take over" warning chime and the car swerving to the right to avoid the side collision."
As of Friday afternoon, the video had more than 1.8 million views.
A video posted to YouTube last October by the same user showed scenarios in which autopilot "might not do so well," according to the commentary.
It added that drivers need to be "very aware of what the car is doing."