An autonomous Uber car that fatally hit a woman had seen the pedestrian six seconds before impact, according to a report.
According to the National Transportation Safety Board (NTSB), the car "failed to stop because the system that automatically applies the brakes had been disabled".
According to US investigators, the driver did not brake until less than a second after it hit 49-year-old Elaine Herzberg in Tempe, Arizona in March.
The car was travelling at 40mph at the time.
The report found that emergency braking manoeuvres are not enabled while the cars are under computer control.
The company uses a human back-up driver to intervene if the brakes need to be applied.
Rafaela Vasquez, 44, was the car's safety driver at the time of the incident.
Ms Herzberg was pushing a bicycle across the road when she was hit by the vehicle.
A video of the crash showed Ms Vasquez looking down just before the crash occurred. In an interview with the NTSB, she said she had been monitoring the "self-driving interface".
While her personal and business telephones were in the vehicle, Ms Vasquez said neither were in use at the time of the crash.
In a statement Uber said that it has worked closely with the NTSB and is doing an internal review of its self-driving vehicle programme.
It has also brought in former NTSB chairman Christopher Hart “to advise us on our overall safety culture, and we look forward to sharing more on the changes we’ll make in the coming weeks.”
Last week, Uber pulled its self-driving cars out of Arizona. Following the March crash, it suspended testing on its self-driving vehicles in Arizona, Pittsburgh, San Francisco and Toronto while regulators investigated.
Daniel Scarpinato, spokesperson for Arizona governor Doug Ducey said that the preliminary report does not provide “any decisive findings or conclusions.”
Additional reporting by Associated Press.
