SAN FRANCISCO – About 21,000 homes and businesses in San Francisco were still without power Sunday morning, a day after a massive outage darkened much of the city during one of the busiest shopping weekends before Christmas, according to PG&E.

What we know:

The utility said the outage began shortly after noon Saturday in the western part of the city before spreading to several neighborhoods, including most of downtown. 

At one point, PG&E estimated that roughly one-third of all San Francisco customers were without electricity.

Investigators are now working to determine whether the outage is connected to a fire Saturday at a PG&E substation near Eighth and Mission streets.

What they’re saying:

“We do not know exactly which happened first, meaning if the fire caused the outage or not,” said San Francisco Fire Department Lt. Mariano Elias. “It’s too early to tell at this time, but in order for us to work there in a safe operation, we need the power off.”

The outage created widespread transportation disruptions throughout the city. With traffic signals dark at major intersections, drivers faced significant congestion in multiple neighborhoods. BART also temporarily closed its Powell Street and Civic Center stations due to the power failure.

The blackout also affected autonomous vehicle service in San Francisco. Waymo temporarily suspended its robotaxi operations, citing safety concerns. Video recorded during the outage showed several of the company’s vehicles stalled in intersections, at times blocking traffic and nearly causing collisions.

In a statement, Waymo said it was “focused on keeping our riders safe and ensuring emergency personnel have the clear access they need to do their work,” adding that it plans to resume service in San Francisco “soon” but did not give a specific time.