Waymo suspended its robotaxi service in San Francisco on Saturday evening after a massive blackout appeared to leave many of its vehicles stalled on city streets.

Numerous photos and videos posted to social media captured Waymo robotaxis stalled at roads and intersections as human drivers were either stuck behind them or weaved around them.

Waymo said on Saturday that it had temporarily suspended service in the city due to the blackout. Spokesperson Suzanne Philion provided a similar statement to TechCrunch on Sunday morning.

“We have temporarily suspended our ride-hailing services in the San Francisco Bay Area due to the widespread power outage,” Philion said. “Our teams are working diligently and in close coordination with city officials to monitor infrastructure stability, and we are hopeful to bring our services back online soon. We appreciate your patience and will provide further updates as soon as they are available.”

The company did not provide an explanation for why the blackout had such a dramatic effect on its vehicles. One possible culprit: The blackout took down many of the city’s traffic lights. (In fact, with the blackout affecting both lights and Muni mass transit, San Francisco Mayor Daniel Lurie warned residents to stay off the roads unless they needed to travel.)

Others theorized that Waymo might have been affected by an interruption in cell service or traffic data.

The blackout appears to have been caused by a fire at a Pacific Gas & Electric substation in the city. SFGate reports that around 120,000 PG&E customers were affected by the blackout, and while the majority of them had power restored by late Saturday, 35,000 customers were still without power on Sunday morning. PG&E’s website also showed thousands of San Francisco customers still affected at that time.

A letter from Tiger Global Management that leaked earlier this month said Waymo is now providing 450,000 robotaxi rides per week, nearly double the amount that the Alphabet-owned company disclosed in the spring.