A Tesla car was easily tricked into driving in Autopilot mode with no one at the wheel, testers from a major US consumer organisation have found.
The findings from Consumer Reports come just days after a Tesla crashed in Texas, killing the two men in the car.
Officials say neither of the men were in the driver's seat at the time of the crash.
The cars' Autopilot partially automated system can keep it centered in its lane, keep a distance from vehicles in front of it, and can even change lanes with a driver's consent.
Tesla says the driver must be ready to intervene at all times.
However, Consumer Reports said that during several trips on its closed tracks with an empty driver's seat, its Tesla Model Y automatically steered along painted lane lines without acknowledging that nobody was at the controls.
The Tesla involved in the crash near Houston over the weekend was a Model S, but also had an Autopilot function.
"In our evaluation, the system not only failed to make sure the driver was paying attention, but it also couldn't tell if there was a driver there at all," said Jake Fisher, Consumer Reports' senior director of auto testing, who conducted the experiment.
"Tesla is falling behind other automakers like GM and Ford that, on models with advanced driver assist systems, use technology to make sure the driver is looking at the road."
Based in Palo Alto, California, Tesla has disbanded its press office and has not responded to enquiries about Consumer Reports' assertions.
The National Highway Traffic Safety Administration and the National Transportation Safety have launched investigations into the Texas crash.
Local authorities said one man was found in the passenger seat, while another was in the back. Officials say the car veered off the road, crashed into a tree and burst into flames.
Investigators should be able to establish whether the Tesla's Autopilot system was in use.
Tesla CEO Elon Musk said on Monday that data logs "recovered so far" show Autopilot was not turned on in the Texas crash, and "Full Self-Driving" was not purchased for the vehicle.
He did not respond to reporters' questions posted on Twitter.