The Tesla electric car driving in semi-autonomous mode when it crashed and killed its driver was speeding just ahead of impact with a tractor-trailer, a preliminary probe showed Tuesday.
The National Transportation Safety Board said its preliminary findings showed the Tesla was traveling at 74 miles (119 kilometers) per hour ahead of impact, but noted that investigators have not yet completed an analysis of the crash data or assigned a cause for the fatal collision.
The car was driving in excess of the posted speed limit of 65 miles per hour before the crash.
"All aspects of the crash remain under investigation," the NTSB said in a statement on its review of the May 7 fatality, which raised concerns about the safety of the rapidly growing field of autonomous driving technology.
The NTSB said the driver was using the advanced driver assistance features called Traffic-Aware Cruise Control and Autosteer lane keeping assistance.
It was also equipped with automatic emergency braking that is designed to automatically apply the brakes to reduce the severity of or assist in avoiding frontal collisions.
Tesla earlier this month acknowledged the crash and said its sensors failed to pick up the white side of the truck against a brightly lit sky.
With the truck crossing the divided highway, the impact sheared off the top of the car and killed the driver.
Some reports said the driver may have been watching a DVD at the time, ignoring Tesla's warning to remain vigilant while using the Autopilot system.
Tesla said the Autopilot system, introduced last year, is not a fully autonomous system and that drivers are cautioned that they need to be at the wheel and in control.
The system allows the vehicle to automatically change lanes, manage speed and brake to avoid a collision. The system may be overridden by the driver.
Backers of autonomous driving say that despite the Tesla fatality, the technology is likely to eliminate a large percentage of accidents, which are attributed mainly to human error.
Most major automakers are also looking at autonomous cars.
BMW has announced that it is joining forces with US computer chip giant Intel and the Israeli technology firm Mobileye to develop self-driving cars, aiming for fully automated driving in production cars by 2021.
South Korea's Kia has pledged to produce a self-driving car by 2020 and General Motors plans to test the technology with ridesharing giant Lyft.
Google has driven its some 1.5 million miles (2.4 million kilometers) with only some minor dust-ups.