The board’s conclusion is the latest development in a string of federal investigations into crashes involving Autopilot, which can, among other things, keep a moving car in its lane and match the speed of surrounding vehicles. Tesla has said that the system should be used only under certain conditions, but some safety experts say the company doesn’t do enough to educate drivers about those limitations or take steps to make sure drivers do not become overly reliant on the system and, thus, distracted.

Mr. Huang had been playing a game on his phone during the drive, but it was not clear whether he was engaged with the game in the moments before the crash, according to the investigation.

The concerns about Autopilot have done little to slow Tesla’s rise. The company’s share price has more than tripled since October as Tesla’s financial performance has surpassed even the rosiest of analyst expectations. In September, Tesla earned its first safety award from the nonprofit Insurance Institute for Highway Safety, and last week Consumer Reports named Tesla’s first mass-market electric car, the Model 3, one of its top picks for 2020.

Tesla has repeatedly said that Autopilot makes its vehicles safer. In the fourth quarter of 2019, the company reported one accident for every three million miles driven in a Tesla with Autopilot engaged. Over all, the national rate was one accident for every 498,000 miles driven in 2017, according to NHTSA.

Still, the electric-car maker faces scrutiny on multiple fronts. The N.T.S.B. and the traffic safety administration are currently investigating more than a dozen crashes in which Autopilot might have played a role.

In the 2018 accident, Autopilot had been engaged for nearly 19 minutes, according to the safety board’s investigation. Mr. Huang put his hands on and off the wheel several times during that period, and in the final minute before the crash, the vehicle detected his hands on the wheel three times for a total of 34 seconds. It did not detect his hands on the wheel in the six seconds before impact.