An Uber self-driving car hit and killed a woman crossing the street in Arizona, police said on Monday, marking the first fatality involving an autonomous vehicle and a potential blow to the technology expected to transform transportation.

The ride services company said it was 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 Inc and General Motors Co , 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.

U.S. 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 10:00pm. MST Sunday (0400 GMT Monday) when she was struck by the Uber vehicle traveling 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.’