The world is on the edge of its seat as it watches autonomous vehicles (AV) take to the roads, anticipating the potential damage they will bring. The United States’ first reported fatal crash involving a self-driving Uber SUV and a pedestrian happened on a Sunday, March 18 in Arizona, resulting in the first pedestrian AV victim.

“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,” tweeted Uber’s CEO Dara Khosrowshahi.

According to investigations by the Tempe Police Department in Arizona, the self-driving car was in autonomous mode at the time of the crash and hit a middle-aged woman, Elaine Herzberg, who was walking outside of the crosswalk. She later died at a hospital as a result of her injuries. A vehicle operator, 44-year-old Rafaela Vasquez, was behind the wheel and reported no injuries; there were no passengers on board at the time of the accident.

In response to this incident, Uber has paused self-driving operations in Phoenix, Pittsburgh, San Francisco, and Toronto, which, according to the company, is a standard response.

Although this is not the first accident involving AVs, this is the first collision to result in the death of a pedestrian. In March 2016, Google’s self-driving car crashed into a bus in California, igniting a debate about the reliability of the tech behind self-driving cars. As in the Arizona case, a safety driver was at the wheel, but did not override the automated system.

In July 2016, an autonomous Tesla Model S crashed into a truck and killed the only passenger on board, prompting the National Highway Traffic Safety Administration (NHTSA) to open an investigation into the performance of the vehicle’s self-driving mode.

Uber moved its AV testing to Tempe, Arizona in February 2017, after driverless cars were banned from California roads due to lack of required permits. Users could try an autonomous Uber in Arizona by requesting an UberX through the app. Governor Doug Ducey himself took an autonomous Uber on Feb 21, 2017.

This incident, which in this case involved a pedestrian and a safety driver, will likely revive the issue of the Trolley Problem in AVs. The Trolley Problem is a thought experiment in the field of ethics. The hypothetical scenario says that there is a runaway trolley barrelling down the railway tracks. Ahead on the tracks, there are five people tied up and unable to move. The trolley is headed straight for them. You, the decision-maker in the story, are standing some distance off in the train yard, next to a lever. If you pull the lever, the trolley will switch to a different set of tracks on which one person tied up. There is a never-ending dilemma over which is the most ethical thing to do: nothing, and the trolley kills the five people on the main track. Or pull the lever, diverting the trolley onto the side track where it will kill one person.