Dana Cronin

PG&E, the city's utility company, is investigating the cause of the outage.

The San Francisco Fire Department responded to a fire Saturday afternoon at a PG&E substation in the city, but says it's unclear whether that was the root cause.

The outage paused transit services and caused traffic jams across the city due to malfunctioning traffic lights and driverless taxis stuck in intersections.

For NPR News, I'm Dana Cronin in San Francisco.