Officials said an Arizona woman was killed after being struck by a self-driving Uber vehicle Sunday — an incident believed to be the first of its kind.

The accident in the Phoenix suburb of Tempe caused the company to suspend all testing of self-driving vehicles in cities across the country.

Tempe Police Sgt. Ronald Elcock told Fox News that Elaine Herzberg, 49, was walking outside of a crosswalk when she was struck by the vehicle just before 10 p.m.

"The vehicle was traveling northbound just south of Curry Rd. when a female walking outside of the crosswalk crossed the road from west to east when she was struck by the Uber vehicle," police said.

Herzberg was taken to a nearby hospital, where she later died, according to police.

The self-driving Uber was in autonomous mode moving at a speed of 40 mph at the time of the collision, Elcock said at a news conference.

The driver, identified as 44-year-old Rafael Vasquez, has been cooperating in the investigation and was not found to have been impaired at the time of the accident, police said.

Uber said on Twitter the company is fully cooperating with local authorities as the investigation occurs, and told Fox News it has halted testing of the self-driving vehicles in cities across the country.

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"Our hearts go out to the victim’s family," Uber Comms tweeted. "We’re fully cooperating with @TempePolice and local authorities as they investigate this incident."

Uber CEO Dara Khosrowshahi tweeted: "Some incredibly sad news out of Arizona. We’re thinking of the victim’s family as we work with local law enforcement to understand what happened."

The company has been testing autonomous vehicles in Pittsburgh, San Francisco, Toronto and the greater Phoenix area for months. Automakers and tech companies are competing to be first with the technology.

The National Transportation Safety Board said it was sending a team to investigate the crash.

The National Highway Traffic Safety Administration said in a statement to Reuters it is "in contact with Uber, Volvo, federal, state and local authorities regarding the incident," and will take appropriate action.

The federal government has voluntary guidelines for companies that want to test autonomous vehicles, leaving much of the regulation up to states.

The U.S. Department of Transportation is considering other voluntary guidelines that it says will help foster innovation. But Transportation Secretary Elaine Chao also has said technology and automobile companies need to allay public fears of self-driving vehicles, citing a poll showing that 78 percent 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.

California is among those that require manufacturers to report any incidents to the motor vehicle department during the autonomous vehicle testing phase. As of early March, the agency received 59 such reports.

Fox News' Nicole Darrah, Shira Bush and Charlie Lapastora, along with The Associated Press, contributed to this report.