The structure is still smoldering, even burning in placesband far too unstable for investigators to enter.
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"We can't get into the burn area until the structural engineer has made his final determination and we've taken action to be as safe as we can be," said Robert Marshall, Contra Costa County Fire Marshal.
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The ATF has arrived with a national response team that includes arson investigators from all over the country, including Los Angeles and San Diego.
There is also a team of structural engineers on site trying to figure out the stability of the structure, and whether it can be shored up or needs to be demolished.
In the meantime, the 250 residents who live in the Renaissance Square apartments next door were allowed in briefly Tuesday to retrieve some belongings, but otherwise remain evacuated.
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"What the fear is," explained Marshall, "Is if the building were to collapse over on that side and it should damage the apartment building there, then it could compromise the structure of the entire apartment building."
Reyes Ramos owns the Agave Grill and Cantina on Willow Pass Rd. His business is a block away from the fire scene itself yet still inside a gated perimeter.
"The damage was pretty much to the outside," said Ramos, pointing to holes in his fabric awnings from burning embers. "We have these barricades and we don't know when they're going to come down, because they're afraid the building is going to come down on Willow Pass Rd. So they're not allowing any traffic and our parking lot is closed right now."