In its blog post responding to the crash, Tesla noted that "neither Autopilot nor the driver noticed the white side of the tractor trailer against a brightly-lit sky". Brown would be the first known fatality connected to the Autopilot feature if the semi-autonomous system is to blame.
We talked to the NTSB in February about its scope, and the agency said then it typically doesn't look into light passenger vehicle crashes the way it would with a plane or train.
"No safety-significant system should ever use consumers as test drivers on the highways", said Clarence Ditlow, head of the nonprofit Center for Automotive Safety.
While the investigation is ongoing, there remains the possibility that the accident was in fact caused by human error.
At least two more crashes involving Tesla vehicles with engaged Autopilot have been reported since the NHTSA made the announcement that the investigation is ongoing.
"My concern is that this was an avoidable accident", Cummings told AFP.
The agency is seeking details of all design changes and updates to Autopilot since it went into use previous year, and information on whether Tesla plans updates in the next four months. "I also don't think it's material, but I didn't know about it".
The attention generated by the fatal Tesla accident has stoked a public debate about the safety and wisdom of making "self-driving cars".
These types of crash-avoidance systems aren't unique to Tesla or Autopilot; many automakers now offer something similar on new cars and trucks. It is in the process of developing standards for self-driving cars.
While Tesla has been the focus of the debate, many cars, including models from BMW, Mercedes-Benz and Volvo, have systems that use a combination of adaptive cruise control, lane-keeping and automatic braking. South Korea's Kia has pledged to produce a self-driving vehicle by 2020 and General Motors plans to test the technology with ride-sharing giant Lyft. "Among all vehicles in the USA, there is a fatality every 94 million miles", the company said on its website last week.
Even when the feature was launch, Tesla said that the feature is there to help the drivers only.
"It says beta specifically so people do not become complacent", Musk underlines.
He said the benefits of the technology are not in doubt, but that it may take some 10 years to deploy a fully autonomous auto. No further details were given as to what Part 2 of the Tesla Masterplan.
A survey earlier this year by the AAA auto club showed 75 percent of United States drivers would be afraid to ride in an autonomous vehicle.