The engineers have two main theories, the people said. Either the car's cameras and radar failed to spot a crossing tractor-trailer. Or the cameras didn't see the rig and the car's computer thought the radar signal was false, possibly from an overpass or sign.
Tesla officials disclosed these theories to U.S. Senate Commerce Committee staff members during an hour-long meeting, said the people, who didn't want to be identified because the meeting was private. The meeting came at the request of Committee Chairman Sen. John Thune, R-S.D., whose committee oversees transportation and could hold a hearing on self-driving technology at a later date.
The driver of the Model S, Joshua Brown of Canton, Ohio, was killed when the sedan hit the side of a tractor-trailer while traveling nine mph above the speed limit on a highway near Gainesville, Florida, federal accident investigators have said. The National Highway Traffic Safety Administration, which is investigating the May 7 crash, has said that the car's Autopilot system was engaged.
Brown, 40, a tech company owner, was using the car's automatic emergency braking and lane-keeping features at the time, a report from the National Transportation Safety Board said. Those features are part of the vehicle's Autopilot system.
The Tesla struck the underside of the truck's 53-foot semitrailer at a 90-degree angle, shearing off the sedan's roof before it emerged on the other side of the trailer, according to the report. The truck was making a left turn on a highway that allows cross traffic.
Tesla is still working to pinpoint what system failures caused the crash and still believes the safety benefits of semi-autonomous driving systems like Autopilot far outweigh any risks, the people said.
Representatives of the Palo Alto, California, company also told committee staffers that cross traffic remains a challenge for automated driving systems, according to the person.
A Tesla spokeswoman confirmed that the meeting took place and said in a statement that the Florida crash fell "within a unique set of circumstances for which the camera and radar were not able to provide the appropriate warning or braking support."
In the briefing, Tesla officials told staffers that radar sensors connected to the automated braking system may have spotted the tractor-trailer, but the Model S computer may be designed to "tune out" overhead structures, such as bridges and highway signs, "to avoid the triggering of false braking events," the person said.
Tesla has said previously that Autopilot was unable to distinguish the white side of the truck from the brightly lit sky and there was no attempt to brake by either the self-driving system or Brown.
Tesla officials wouldn't answer questions about the company's relationship with Mobileye, the company that made the camera and computer system for Tesla's automatic braking, one of the people said. Mobileye said earlier this week that it would end its relationship with Tesla. The company previously has said its system was not designed to spot cross traffic, but it didn't know if Tesla modified the system.
Tesla CEO Elon Musk has said Tesla will press ahead with semi-autonomous driving features, which he says will prevent injuries and accidents.
The company says it tells drivers that they must continue to pay attention while Autopilot is working and be ready to retake control of the vehicle.