SAN FRANCISCO — More than a second before a self-driving car operated by Uber struck and killed a pedestrian in March, the vehicle’s computer system determined it needed to brake to avoid a crash. But a built-in emergency braking system had been disabled while the car was in autonomous mode to ensure a smoother ride, according to the preliminary report of federal regulators investigating the crash.

The initial findings, released on Thursday by the National Transportation Safety Board, confirmed early police reports about the episode, which was the first known pedestrian fatality involving an autonomous car.

The report also affirmed what many experts on self-driving cars said in the days after the crash: Uber’s cars are loaded with sensors and cameras that should have detected a pedestrian with plenty of time to stop. But this one failed through a combination of a computer system not responding properly to the pedestrian’s presence and a distracted safety driver.

The fatal crash has raised questions about the safety of testing self-driving cars on public roads and whether companies, in their rush to be among the first to deploy this breakthrough technology, are taking enough precautions to prevent harm to other drivers and pedestrians. It has also dampened some enthusiasm for self-driving cars as a technology that researchers believe could eventually save lives because robot drivers, unlike humans, don’t get distracted or tired and obey traffic rules.