Uber has suspended testing of its self-driving cars after one struck and killed a female cyclist in Phoenix.
Investigators said the vehicle was in autonomous mode with an operator behind the wheel when the woman, 49-year-old Elaine Herzberg, was struck.
She died from her injuries in hospital.
In a statement on Twitter, Uber said: "Our hearts go out to the victim's family. We are fully co-operating with local authorities in their investigation of this incident."
The company later said it was suspended its self-driving car tests in Phoenix, Pittsburgh, San Francisco and Toronto.
Uber chief executive Dara Khosrowshahi also expressed condolences on his Twitter account, describing it as "incredibly sad news".
The ride-hailing firm has been testing the self-driving vehicles in Tempe and Phoenix for months.
In 2017 it temporarily suspended its vehicles in Arizona after a crash involving one of its cars, a Volvo SUV.
Its vehicles ran into trouble in 2016 when some were found to be jumping red lights.
The government has voluntary guidelines for companies that want to test autonomous vehicles, leaving much of the regulation up to states.
The US Department of Transportation is considering other voluntary guidelines that it says will help encourage innovation.
But Transportation Secretary Elaine Chao has also said technology and car companies need to allay public fears of self-driving vehicles.
She highlighted a poll showing that 78% of people fear riding in autonomous vehicles.
The number of states considering legislation related to autonomous vehicles gradually has increased each year, according to the National Conference of State Legislatures. In 2017 alone, 33 states introduced legislation.
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California is among those that require manufacturers to report any incidents to the motor vehicle department during the autonomous vehicle testing phase.
By early March, the agency received 59 such reports.