Tesla's design of its Autopilot system contributed to a 2016 fatal crash in Florida, U.S. accident investigators concluded.

The National Transportation Safety Board, in its first probe of autonomous driving technologies being introduced by carmakers, recommended that systems such as Telsa's Autopilot be unavailable when the vehicle is traveling on a road where its use is inappropriate, the NTSB concluded in Washington Tuesday.

The accident occurred on a road that wasn't a limited access highway for which Autopilot is designed.

The NTSB said the Autopilot system operated as designed but did not do enough to ensure drivers paid adequate attention. On some roads, drivers could use Autopilot at up to 90 miles per hour, it said.

Tesla did not ensure that the system was only used on highways and limited-access roads, as recommended in the owner's manual, a fact that NTSB Chairman Robert Sumwalt noted.

"Tesla's system worked as designed," Sumwalt said. "Tesla allowed the driver to use this system in a way it wasn't designed."

Joshua Brown, a former Navy SEAL, died May 7, 2016 when his Model S struck a truck crossing the road in front of him on a Florida highway.

The board also recommended better data sharing by automakers and for improvements in the way vehicles ensure drivers are paying attention.