The "operational limitations" of Tesla's Autopilot system played a "major role" in a 2016 crash, the National Transportation Safety Board said on Tuesday.
NTSB investigators met to determine the cause of a fatal crash involving a Tesla on Autopilot in May 2016.
After meeting for roughly 2½ hours Tuesday, the board determined the probable cause of the accident was a combination of a semitrailer driver failing to yield the right of way to the Tesla driver, the Tesla driver's overreliance on the car's Autopilot system, and certain design limitations in the Autopilot system that failed to adequately warn the Tesla driver of the approaching truck.
They recommended that automakers not allow drivers to use automated control systems in ways they were not intended to be used. The driver in the fatal Tesla crash had been using Autopilot on a type of road Tesla said Autopilot should not be used on.
"At Tesla, the safety of our customers comes first, and one thing is very clear: Autopilot significantly increases safety, as NHTSA has found that it reduces accident rates by 40 percent," Tesla said in an email to CNBC.
"We appreciate the NTSB's analysis of last year's tragic accident and we will evaluate their recommendations as we continue to evolve our technology," Tesla wrote. "We will also continue to be extremely clear with current and potential customers that Autopilot is not a fully self-driving technology and drivers need to remain attentive at all times."
The board previously determined the crash was not the result of a defect in the system.
The accident was the first known fatal crash involving a car using an automated driver assistance system, NTSB chairman Robert Sumwalt said.
The vehicle was set to cruise control at 74 miles per hour, above the 65 mph speed limit, moments before the crash.
The Autopilot system failed to detect a semitrailer crossing an intersection in front of the car, and the car collided with the truck, killing the driver.
The driver had Autopilot engaged for 37 of the 41 total minutes of the trip.
Autopilot has torque sensors on the steering wheel that detect whether the driver is holding it. If the sensors indicate the driver is not holding the wheel, it issues warnings that will disengage Autopilot, forcing the driver to take over again.
Data taken from the car indicate the driver had his hands on the wheel seven times during the time the system was engaged, for a total for 25 seconds.
The driver's "lack of engagement" suggests an over-reliance on the Autopilot system, NTSB investigators said.
In particular, the investigators said using torque sensors on the steering wheel is a poor method for gauging driver engagement. Driving is a largely visual activity, said NTSB investigator Ensar Becic, and whether hands are on the wheel does not necessarily indicate whether the driver is paying attention.
The board recommends companies turn to other technologies.
"One potential option is an eye tracker, a driver-facing camera," Becic said.
— Reuters contributed to this report.