A self-driving Uber hit and killed a woman on Sunday in Tempe, Arizona, according to USA Today. This is the first reported death involving self-driving cars, and the accident could become a setback in the development of autonomous vehicles.

A safety driver was sitting in the front seat of the vehicle while the car’s self-driving mode was being tested by Uber, according to local police. Uber has since issued a statement saying that the company is "fully cooperating with local authorities,” and that it will be temporarily stopping its self-driving tests in Pittsburgh, San Francisco, and Toronto.

The identity of the woman killed in the accident has not been released, but police say she was walking on a designated crosswalk around 10 p.m. on Sunday night when she was hit by the vehicle.

In response to the incident, Uber’s CEO Dara Khosrowshahi tweeted on Monday that he is “thinking of the victim’s family as we work with local law enforcement to understand what happened."

Since this is the first known death that involves a self-driving vehicle, the accident could help fuel concerns that the technology is not yet up to par. Uber isn’t the only company testing autonomous cars. Waymo, the name of Google's self-driving car company, and other automakers have been testing self-driving cars for years in an attempt to reduce consumer deaths caused by motor vehicle accidents. As USA Today points out, 40,000 people were killed in car accidents last year.

It’s unclear if this accident will slow down the pace of development on autonomous vehicles, but either way, this is a blow for Uber, a company already clouded by negative press and known for underpaying its employees.