A woman in Tempe, Arizona, has died after being hit by a self-driving Uber vehicle.

ABC 15 Arizona reports that the Uber vehicle was operating in autonomous mode with a human behind the wheel when the incident occurred overnight. The woman, who has not yet been identified, was reportedly struck while walking outside of a crosswalk. She was taken to the hospital and later died from her injuries.

Uber CEO Dara Khosrowshahi posted about the incident on Twitter:

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. https://t.co/cwTCVJjEuz — dara khosrowshahi (@dkhos) March 19, 2018

"Our hearts go out to the victim's family," an Uber spokesperson told PCMag in an email. "We are fully cooperating with local authorities in their investigation of this incident."

The company has halted its self-driving car tests in Tempe, Pittsburgh, San Francisco, and Toronto.

As The New York Times notes, this appears to be the first time an autonomous vehicle has killed a pedestrian on public roads.

Uber has been testing its autonomous vehicles in Tempe since February 2017. Around this time last year, a Volvo SUV fitted with Uber self-driving tech collided with another vehicle that failed to yield, causing significant damage but no serious injuries. Following that collision, Uber temporarily suspended its US self-driving programs in Tempe, Pittsburgh, and San Francisco.

Meanwhile, Google's autonomous test vehicles have been involved in several crashes over the years, including an incident in September 2016 when another car ran a red light and collided with a Google SUV. That same year, a more serious crash left a Tesla driver dead after his car, operating in "Autopilot," collided head-on with a truck.