Uber temporarily hit the brakes on tests of its self-driving cars following a crash Friday evening in Tempe, AZ., that involved one of the company’s autonomous vehicles. Tests vehicles in Arizona, Pittsburgh and San Francisco were grounded over the weekend while the investigation continued.

“We are continuing to look into this incident and can confirm we had no backseat passengers in the vehicle,” an Uber spokeswoman emailed USA TODAY on Sunday. The vehicle involved in the accident was in a self-driving mode.

There are no known serious injuries.

On Monday, Uber emailed an updated statement that, “We are resuming our developmental operations in San Francisco this morning.” For now, though, test vehicles in Arizona and Pittsburgh remain on the sideline, though Uber hopes to get those vehicles back on the road soon as well.

Uber has been testing self-driving Volvos, as part of a $300 million partnership with the Swedish automaker.

This has been anything but a smooth ride for Uber. Last month, rival Alphabet Inc.’s self-driving car company Waymo sued Uber claiming that a former Google car executive Anthony Levandowski stole critical laser sensor technology, shortly before leaving the company in order to start Otto, a company Uber bought last August for $670 million. Uber called it a “baseless” ploy to slow the progress of a competitor.

Uber also got into a tiff with the California Department of Motor Vehicles back in December when the DMV revoked the vehicles’ registrations in a dispute over whether testing permits were required. At the time, Arizona’s GOP Governor Doug Ducey said he “welcomed Uber self-driving cars with open arms and wide open roads. While California puts the brakes on innovation and change with more bureaucracy, Arizona is paving the way for new technology and new businesses.