A white semitrailer on a bright day proved invisible to the self-driving system of a Tesla S sedan, which failed to brake and ran under the trailer as it crossed in front of the car, said authorities investigating the first fatal crash of an autonomous car in the United States, which happened in early May.

The accident killed 40-year-old Joshua Brown, of Canton, Ohio, on a clear, dry roadway on May 7 in Williston, Fla., according to the Florida Highway Patrol. It’s expected to intensify a debate within the automotive industry and in legal circles over the safety of systems that take partial control of steering and braking from drivers.

The National Highway Traffic Safety Administration said preliminary reports indicate the crash occurred when a tractor-trailer made a left turn in front of the Tesla at an intersection. Tesla said in a blog post on June 30 that "neither autopilot nor the driver noticed the white side of the tractor-trailer against a brightly lit sky, so the brake was not applied."

Brown’s car under-rode the trailer, moved slightly to the right and ran off the road, then continued through two fences and swerved left, coming to rest against a pole, CBS News reported July 1.

Ironically, Brown had posted videos on YouTube showing how well his advanced electric-drive Tesla’s self-driving system worked; so have numerous other Tesla owners.

In its blog post, Tesla explained how drivers can initiate the autopilot feature in its Model S cars and stated that the technology is "still in a public beta phase," and that contrary to videos posted by users, drivers are not supposed to be operating hands-free. "When drivers activate Autopilot, the acknowledgment box explains, among other things, that Autopilot 'is an assist feature that requires you to keep your hands on the steering wheel at all times,' and that 'you need to maintain control and responsibility for your vehicle' while using it."

The truck driver, Frank Baressi, 62, of Palm Harbor, Fla., told a reporter that Brown was “playing Harry Potter on the TV screen” at the time of the crash and that "he went so fast through my trailer I didn’t see him.”

“It’s a warning that drivers need to be vigilant, even if a self-driving feature is engaged,” said the spokesman for the Insurance Institute for Highway Safety, Russ Rader. “It also serves as a notice that these autonomous vehicles are not just around the corner as something we can buy.”

Although IIHS has campaigned for more effective rear impact guards on trailers, Rader did not criticize the trailer’s apparent lack of side under-ride guards. These are required on trucks and trailers in several other countries, but not in the U.S. Some safety groups are arguing for them.

Most van-type semitrailers are painted white, a practical color that’s easy to see under most conditions, but evidently not those on the day of the Tesla crash. It’s now possible that safety advocates will petition for high-visibility markings, in addition to the red-and-white taping now required on semitrailers.

The crash is the first well-reported incident baring glitches in autonomous systems’ controls, possibly involving software as well as cameras and sensors.

Positive reports on the safety of self-driving cars, including Google’s test vehicles operating in California, have cast a favorable glow on development of autonomous cars and trucks, and suggested adoption is not far off. This incident might change that, observers say, and groups like Consumer Watchdog are demanding that NHTSA issue rules requiring long, careful testing.

Developers of control systems for commercial trucks have been cautious in their statements, saying that many steps will be required and that autonomous trucks might be 10 years away from routine operations.