Joshua Brown, the Tesla owner who was the first known driver to be killed while using the car’s Autopilot technology, highlighted one of its problems in a YouTube post in April. His 2015 Tesla Model S, he said, didn’t recognize stopped vehicles.

“The car sees moving vehicles GREAT,” he posted. “Even really slowly moving vehicles. It has a harder time with stopped ones. I suspect this is due to how radar works.”

The next month, however, on May 7, Mr. Brown was driving along U.S. Route 27 in Williston, Fla., when a 2014 semi tractor-trailer made a turn in front of him, according to a police report. Mr. Brown’s car then plowed under the truck, the police report said, before hitting two fences and spinning around.

It is unclear how fast Mr. Brown’s car was moving, but Tesla Motors Inc. confirmed it was running on Autopilot.

“The Tesla did not put on the brakes,” Frank Baressi, driver of the truck, said in an interview.