“Our hearts go out to the victim’s family,” a company spokeswoman said in a statement. “We are fully cooperating with local authorities in their investigation of this incident.”

The accident in the city of Tempe marked the first fatality from a self-driving vehicle, which are still being tested around the globe, and could derail efforts to fast track the introduction of the new technology in the United States.

“The vehicle was travelling northbound … when a female walking outside the pedestrian crossing crossed the road from west to east when she was struck by the Uber vehicle,” police said in a statement.

As many self-driving cars have begun to travel on public roads in pilot programmes around the world, the chance of a pedestrian death has become more likely.

Experts have long worried about the impact deadly crashes could have on the nascent industry.

“We’re within the phase of autonomous vehicles where we’re still learning how good they are. Whenever you release a new technology there’s a whole bunch of unanticipated situations,” said Arun Sundararajan, a professor at New York University’s business school. 

“Despite the fact that humans are also prone to error, we have as a society many decades of understanding of those errors.”

The National Transportation Safety Board is opening an investigation into the death and is sending a small team of investigators to Tempe. The Department of Transportation’s National Highway Traffic Safety Administration (NTSB) dispatched a special crash investigation team.

The NTSB opens relatively few highway accident investigations each year, but has been closely following incidents involving autonomous or partially autonomous vehicles. 

Last year, it partially faulted Tesla’s Autopilot system for a fatal crash in Florida in 2016.

The NTSB’s cautionary tone on the emergence of self-driving technology contrasted with the Department of Transportation, which revised its policy on self-driving vehicles Tuesday in an attempt to remove obstacles to the testing of such vehicles.

“As always we want the facts, but based on what is being reported this is exactly what we have been concerned about and what could happen if you test self-driving vehicles on city streets,” said Jason Levine, executive director of the Centre for Auto Safety, a Washington-based advocacy group. 

“It will set consumer confidence in the technology back years if not decades. We need to slow down.”

The Phoenix area is a fertile ground for experiments in the technology. Uber has been testing there with safety drivers behind the wheel. 

Late last year, Alphabet’s Waymo, which has tested in the Phoenix area for years, began removing the safety drivers to transport a small number of residents.

General Motors is also testing in the Phoenix area. A GM spokesman declined to comment, and a representative from Waymo did not return multiple requests for comment.

“Public safety is our top priority, and we are in communication with law enforcement, which is investigating the accident and gathering facts, as well as Uber,” a spokesman for Arizona Governor Doug Ducey said.

Drivers relying on Tesla’s Autopilot technology have been involved in fatal car crashes. Uber has had minor incidents in the past. 

A self-driving Uber car ran a red light in San Francisco while the company operated in the city without regulatory approval. The California Department of Motor Vehicles eventually forced Uber to pull the cars from the road.

With additional reporting from Associated Press