The NTSB said it tested a different Tesla vehicle on the same road, and the Autopilot driver-assist system could not be fully used. Investigators could not get the system's automated steering system to work, but were able to use Traffic Aware Cruise Control.
Autopilot needs both the cruise control and the automatic steering to function. Traffic Aware Cruise Control can keep the car a safe distance from vehicles in front of it, while autosteer keeps it in its own lane. The report said the road also did not have lane lines. That could have been why the automatic steering wouldn't work.
"The NTSB continues to collect data to analyse the crash dynamics, postmortem toxicology test results, seat belt use, occupant egress and electric vehicle fires," the agency said in its report.
"All aspects of the crash remain under investigation as the NTSB determines the probable cause."
The agency says it intends to issue safety recommendations to prevent similar crashes.
The April 17 crash happened at 9.07pm on Hammock Dunes Place, a two-lane residential road in Spring, Texas. Both the 59-year-old owner and the 69-year-old passenger were killed.
The NTSB report said the 2019 Model S went off the road on a curve, drove over a curb, hit a drainage culvert, a raised manhole and a tree.
The crash damaged the high-voltage lithium-ion battery, where the fire began. Firefighters used 120,000 litres of water over four hours to put out the flames because the car's batteries kept reigniting.
Local authorities said one man was found in the front passenger seat, while another was in the back.
The report didn't say how fast the car was going, but Harris County Precinct Four Constable Mark Herman said it was a high speed. He would not say if there was evidence anyone tampered with Tesla's system to monitor the driver, which detects force from hands on the steering wheel.
The system will issue warnings and eventually shut the car down if it doesn't detect hands. But critics say Tesla's system is easy to fool and can take as long as a minute to shut down.
Consumer Reports said in April that it was able to easily trick a Tesla into driving in Autopilot mode with no one at the wheel.
The NTSB, which has no regulatory authority and can only make recommendations, said it's working with the National Highway Traffic Safety Administration on the probe. NHTSA has the power to make vehicle safety regulations. The federal probe is running at the same time as a parallel investigation by local authorities, the NTSB said.
The Texas crash raised questions of whether Autopilot was working at the time, and whether Tesla does enough to make sure drivers are engaged. The company says in owner's manuals and on its website that Autopilot is a driver-assist system and drivers must be ready to take action at any time.
Lars Moravy, Tesla's vice president of vehicle engineering, said on the company's April 26 earnings conference call that an inspection of the badly burned car found that the steering wheel was deformed, "so it was leading to a likelihood that someone was in the driver's seat at the time of the crash". He said all seat belts were found unbuckled.
Last month on Twitter, Tesla CEO Elon Musk wrote that data logs "recovered so far" in the crashed show Autopilot wasn't turned on, and "Full Self-Driving" was not purchased for the vehicle in the Texas crash. He didn't answer reporters' questions posed on Twitter.
In the past, NHTSA has taken a hands-off approach to regulating partial and fully automated systems for fear of hindering development of promising new features.
But since March, the agency has stepped up inquiries into Teslas, dispatching teams to three crashes. It has investigated 28 Tesla crashes in the past few years, but thus far has relied on voluntary safety compliance from auto and tech companies.
At least three people have been killed in US crashes in which Autopilot was operating but neither the system nor the driver took action to avoid obstacles.
