Newsom announced the finding after analysis of core samples taken from the freeway, a vital artery that hundreds of thousands of vehicles use daily. About 100 columns were damaged in the Saturday fire that officials say was started by arson. No arrests have been made and the investigation is ongoing.

“I don’t want to see that five-week mark hit,” Newsom told reporters as he outlined the repair plan.

Newsom said Monday that investigators are trying to determine if more than one person was involved in the arson but gave no other details.

“I have to stress that we have determined what started the fire,” he said.

Critical questions still unanswered include whether the structure can be repaired or must be knocked down and rebuilt, and how long it will take to reopen the stretch of freeway that normally carries 300,000 vehicles a day.

The fire erupted at 12:20 a.m. Saturday in two storage lots under an elevated section of Interstate 10 where pallets, vehicles and other materials combusted quickly and the fire spread over 8 acres. No injuries were reported but at least 16 homeless people living under the freeway were taken to shelters.

The blaze left many columns charred and chipped and the deck guardrails twisted. Crews shored up the most damaged section for the safety of workers clearing the debris.

The I-10 runs east-west across the heart of the metropolis and connects with other major freeways. Monday brought the first weekday test for commuters navigating new freeway routes or surface street detours around the closed section south of downtown.

The result was both encouraging and “a reminder of the significant challenges we face for the foreseeable future until the 10 can be safely reopened,” Laura Rubio-Cornejo, general manager of the city Department of Transportation, said as Tuesday’s commute began.

There was little congestion during the Monday morning commute, indicating drivers heeded warnings, but the evening commute was much more congested, Rubio-Cornejo said. Surface street detours had a 14.7% increase in traffic volume and traffic engineers adjusted signal patterns to deal with the load, she said.

Freeway closure could affect ports

Beyond a massive traffic headache, the closure is expected to be felt well beyond the metropolis, including possibly slowing the transportation of goods from the twin ports of LA and Long Beach, federal officials said. The ports handle more than half of the goods coming into the country. President Joe Biden was briefed on the fire.

Officials have said the damage is reminiscent of the 1994 Northridge earthquake that crumpled elevated sections of I-10. It took more than two months to repair I-10 after the quake, and that was considered significantly fast.

Newsom said early tests show the deck “appears to be much stronger than originally assessed.” Concrete and rebar samples taken Monday from the superstructure, decks and columns will help determine “whether or not we’re tearing this down and replacing it, or we’re continuing the recovery and repairs,” he said.

“This isn’t going to be resolved in a couple of days, and it’s not going to take a couple years,” Federal Highway Administrator Shailen Bhatt told The Associated Press. “But whether it’s weeks or months, we’re still too early to tell.”

Bhatt said the fiery June 11 crash of a tractor-trailer hauling gasoline in Philadelphia that collapsed an elevated section of Interstate 95, snarling traffic and hurting area businesses, highlights the impact of such disasters nationwide.

“The ports are still open and the goods will still flow, but when you remove a section of the interstate that carries 300,000 vehicles a day, there’s going to be spillover impacts,” Bhatt said.

The closure also raised concerns for downtown businesses.

“Our businesses are just bouncing back from the COVID shutdowns. Business was just getting good,” said Blair Besten, director of LA’s Historic Core business improvement district.

California Fire Marshal Daniel Berlant said investigators have identified where the fire started and what caused it after sorting through the rubble for evidence, but did not specify what they found. He had no information on a suspect and that investigators are talking to witnesses, including homeless people and nearby business owners.

Storage yards under highways are common statewide, with the money from the leases going to public transit. Newsom said the practice would be reevaluated following the fire.

The governor said California has been in litigation with Apex Development, Inc., the owner of the business leasing the storage property where the fire started. The lease has expired, Newsom said, and the business had been in arrears while illegally subleasing the space to five or six other entities.

“They’ve been out of compliance for some time, that’s why we’re going to court” early next year, Newsom said.

Mainak D’Attaray, an attorney for Apex Development, confirmed the lawsuit in an email to The Associated Press.

“We are currently investigating ourselves what happened at the yard under the freeway. As such, we are not prepared to give an official statement or answer questions until we have determined what actually occurred,” D’Attaray said.

Ertugrul Taciroglu, chair of the civil and environmental engineering department at the University of California, Los Angeles, said part of the challenge is how expensive real estate has become.

“Every piece of land is being utilized, so I can see the pressure or the incentives for making use of these spaces under these highways,” he said.

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Watson reported from San Diego. Associated Press writer Christopher Weber in Los Angeles contributed to this report. McMurray reported from Chicago.