An Uber car in self-driving mode has killed a woman in Tempe, Arizona. Details remain scarce and initial accounts vary as to whether the victim was walking or biking at the moment of impact. A human was behind the wheel but was not driving when the collision occurred, according to Uber.

This marks the first time a car in self-driving mode has killed someone outside the vehicle. But the detection of people walking or biking is a known weak point in the development of autonomous cars.

Arizona is one of the major testing grounds for self-driving cars, in part because the state has wide roads with relatively few pedestrians, which are easier for autonomous cars to navigate, though automotive companies are also testing the vehicles in more complex environments. Uber is suspending its autonomous vehicle testing in Tempe, Pittsburgh, Toronto, and San Francisco.

Now that someone has been killed by a self-driving car, questions that seemed like hypotheticals are all too real. Will Uber be held liable for this loss of life? Can these vehicles be safely tested in urban environments where people get around by walking and biking?