Arson was to blame for a blaze underneath a Los Angeles freeway, causing a traffic nightmare by indefinitely shutting down Interstate 10, an artery that carries 300,000 vehicles a day south of downtown in the second-largest US city.

“We know that the origin of this fire is arson,” Los Angeles Mayor Karen Bass said at a news conference Monday.

A mile-long stretch of the freeway has been closed since Saturday in both east and west directions, and drivers are urged to divert north of the city center rather than flood surface streets.

In addition to office workers, the freeway system provides vital links for trucks delivering goods from North America’s largest port complex as holiday season sales are kicking into high gear.

Preliminary samples of the charred freeway section — about 400 feet in length supported by approximately 100 columns — indicated the “structural integrity appears to be stronger than originally assessed,” California Governor Gavin Newsom said at the press conference.

That might allow the damaged section to be repaired rather than completely replaced, meaning the roadway may return to service more quickly than initially thought, he said.

“We’re working our tails off to get this reopened,” Newsom said.

Officials haven’t determined if one or more people started the blaze, according to state Fire Marshal Daniel Berlant.

Bass compared the shutdown of the freeway to such past incidents as the 2011 closure of Interstate 405, which was dubbed Carmageddon, and damage to miles of interstate highways from the 1994 Northridge earthquake.

“Losing this stretch of the 10 freeway will take time and money from people’s lives and businesses,” she said at a briefing earlier in the day. “It’s disrupting in every way.”

Bass urged citizens to use public transit or work remotely to avoid clogging up the center of the city.

The freeway shutdown comes as downtown Los Angeles continues to struggle to recover from the pandemic, which emptied offices, hotels and stores while homeless encampments spread.

“I know we’ve spent this time trying to encourage people to come back downtown, back into their offices,” Bass said on Sunday. “But while we are going through this crisis, we would like for employers who can have their staff work remotely to do so.”

Downtown LA’s office vacancy rate climbed to 25% in the third quarter, up from 21% a year earlier, according to a report by the Downtown Center Business Improvement District.

Tenants have vacated almost 1 million additional square feet of space this year, prompting some landlords, including Brookfield Asset Management, to walk away from buildings with dim financial prospects. The vacancy rate for retail properties climbed to 8.2% from 6.2% a year earlier, the business group reported, as more stores and restaurants closed.

Newsom declared a state of emergency on Saturday to unlock resources aimed at repairing the freeway.

Read More: Resurrection of I-95 in Just Two Weeks Is Dubbed ‘Small Miracle’

In June, a vehicle fire led to the collapse of an Interstate 95 bridge in Philadelphia, creating traffic challenges for a major artery along the East Coast. The six-lane stretch reopened 12 days later after a coordinated response by the federal, state and city governments.

Newsom cited the I-95 incident as a model of a rapid repair.

“But don’t compare the two,” the governor said Sunday. “The scale of this is substantially greater.”

--With assistance from Laura Curtis.

(Updates with fire cause in first paragraph)