An investigation into a fatal crash involving a Tesla Model S autopilot system has safety regulators warning drivers to not use semi-autonomous cars as if they were fully self-driving. The investigation began after a driver using autopilot in a 2015 Tesla Model S died when the car failed to spot a tractor trailer crossing its path. The National Highway Traffic Safety Administration (NHTSA) blamed the driver of the vehicle because he ignored the manufacturer’s warnings to maintain control even while using the driver-assist function. The NHTSA said it found no defects in the vehicle and would not issue a recall.

Just last year, the NHTSA released guidelines to ensure vehicle safety without slowing the development of semi-autonomous and self-driving cars. The agency says self-driving features could dramatically reduce traffic deaths by eliminating human error, which plays a role in 94 percent of fatal crashes. Although Tesla has maintained that autopilot was not responsible for the drivers death, it issued a number of over-the-air updates to the software to increased use of radar sensors and have added a feature that would disable autopilot if drivers took their hands off the wheel too many times.