SAN FRANCISCO (BCN) — San Francisco officials say a major power outage Friday that snarled city traffic and services for much of the day has raised concerns about the integrity of PG&E’s aging infrastructure and the city’s emergency preparations.

A joint letter sent to PG&E CEO Geisha Williams Monday by City Administrator Naomi Kelly, Emergency Management Chief Anne Kronenberg and Fire Chief Joanne Hayes-White asked the utility for more information about the condition of its infrastructure and the status of plans to make repairs and upgrades.

The outage, which affected around 88,000 customers, 21 schools and over 300 traffic lights, shut down the Montgomery BART Station and left some hospitals operating on emergency power backup systems.

Emergency service providers said they responded to more than 100 calls in about two hours after the outage, including 20 different reports of people trapped in elevators.

The outage was caused by a fire at PG&E’s Larkin Street Substation, which city officials said was known to be vulnerable due to aging infrastructure.

“This highlights concerns about the integrity of other PG&E assets,” the letter read. “A service interruption at this scale is more than an inconvenience. It is a failure that affects the lives of residents, businesses, and visitors alike.”

The letter asks for a tour of the substation, information on plans to upgrade it, and a review of other substations as well as backup and recovery plans.

Supervisors London Breed and Aaron Peskin Tuesday also called for a hearing on the power outage focusing on the city’s response to the outage.

“If we look at Friday as a test of our city’s response to a massive power outage and how our agencies collaborated and responded, I would have to say we failed,” Breed said. “Too little information was shared with

too few city agencies, too many people were stranded.”

Breed said it was fortunate no one was injured and there is no evidence the outage was the result of criminal or terrorist activity.

“We got off easy this time, but next time it might be another story,” Breed said.