Arizona police on Monday said an Uber self-driving car hit and killed a woman crossing the street, making it the first fatality involving an autonomous vehicle.

This is a potential blow to the technology that is expected to transform the transport industry, with Uber saying it is suspending North American tests of its self-driving vehicles, which are currently going on in Arizona, Pittsburgh and Toronto.

So-called robot cars, when fully developed by companies including Uber, Alphabet and General Motors, are expected to drastically cut down on motor vehicle fatalities and create billion-dollar businesses.

But Monday’s accident underscored the possible challenges ahead for the promising technology as the cars confront real-world situations involving real people.

US lawmakers have been debating legislation that would speed introduction of self-driving cars.

“This tragic accident underscores why we need to be exceptionally cautious when testing and deploying autonomous vehicle technologies on public roads,” said Democratic Senator Edward Markey, a member of the transportation committee, in a statement.

Elaine Herzberg, 49, was walking her bicycle outside the crosswalk on a four-lane road in the Phoenix suburb of Tempe about 10pm MST Sunday (0400 GMT Monday) when she was struck by the Uber vehicle travelling at about 40 miles per hour (65 km per hour), police said. The Volvo XC90 SUV was in autonomous mode with an operator behind the wheel.

Herzberg later died from her injuries in a hospital, police said.

“The pedestrian was outside of the crosswalk. As soon as she walked into the lane of traffic she was struck,” Tempe Police Sergeant Ronald Elcock told reporters at a news conference. He said he did not yet know how close Herzberg was to the vehicle when she stepped into the lane.

Elcock said he believed Herzberg may have been homeless.

The San Francisco Chronicle late Monday reported that Tempe Police Chief Sylvia Moir said that from viewing videos taken from the vehicle “it’s very clear it would have been difficult to avoid this collision in any kind of mode (autonomous or human-driven) based on how she came from the shadows right into the roadway.”

Moir told the Chronicle, “I suspect preliminarily it appears that the Uber would likely not be at fault in this accident,” but she did not rule out that charges could be filed against the operator in the Uber vehicle, the paper reported.

The “Tempe Police Department does not determine fault in vehicular collisions,” the department said in a statement late Monday, in reply to questions from Reuters about the chief’s comments. “Ultimately the investigation will be submitted to the Maricopa County Attorney’s Office for review and any potential charges.”

Tempe authorities and federal officials are still investigating the incident. Canada’s transportation ministry in Ontario, where Uber conducts testing, also said it was reviewing the accident.

Volvo, the Swedish car brand owned by China’s Geely, said the software controlling the car in the crash was not its own.

Video footage will aid the ongoing investigation, and the case would be submitted to the district attorney, Elcock said.

“Our investigators have that information, and they will be using that in their investigation as well as the Maricopa County Attorney’s Office as part of their investigation,” said Elcock. “They are going to attempt to try to find who was possibly at fault and how we can better be safe, whether it’s pedestrians or whether it’s the vehicle itself.”

–Wild West–

Uber and Waymo on Friday urged Congress to pass sweeping legislation to speed the introduction of self-driving cars into the United States. Some congressional Democrats have blocked the legislation over safety concerns, and Monday’s fatality could hamper passage of the bill, congressional aides said Monday.

Safety advocates called for a national moratorium on all robot car testing on public roads.

“Arizona has been the wild west of robot car testing with virtually no regulations in place,” said Consumer Watchdog, a non-profit consumer advocacy group, in a statement. “That’s why Uber and Waymo test there. When there’s no sheriff in town, people get killed.”

Arizona has opened its arms to companies testing self-driving vehicles as a means to economic growth and jobs. Republican Governor Doug Ducey reached out to Uber in 2016 after California regulators cracked down on the company over its failure to obtain testing permits.

Self-driving cars being tested routinely get into fender-benders with other vehicles. Last week, a self-driving Uber crashed with another vehicle in Pittsburgh, local news reported. There were no injuries.

A year ago, Uber temporarily grounded its self-driving cars for a few days following a crash with another car in Tempe. The company has been the subject of a number of complaints about its autonomous vehicles, but the company has said the cars were being driven by a human driver at the time of the incidents.

–Essential to Uber’s success–

Uber has said its ability to build autonomous cars is essential to its success in the rapidly changing transportation industry. The company envisions a network of autonomous cars that would be summoned through the Uber app that would supplement – and eventually replace – human-driven cars.

Uber has logged 2 million self-driving miles (3.2 million km) through December. The company has more than 100 autonomous cars testing on the roads of the greater Phoenix area, the company’s prime testing ground due to the state’s loose regulations and hospitable weather. Rain, snow and ice are particularly challenging for autonomous cars. The company also tests in Pittsburgh and Toronto.

Concerns over the safety of autonomous vehicles flared after a July 2016 fatality involving a Tesla automobile with a partially autonomous system that required human supervision. Safety regulators later determined Tesla was not at fault.