A structural engineer has decreed that the Walgreens building at 310 University Ave. in downtown Palo Alto is too dangerous for even fire investigators to enter, but officials say that evidence found so far indicates probable arson.

Investigators could be lowered into the building by crane, or a supportive frame could be placed around the building shell, which could be costly, according to fire and police officials.

Sections of the building may be cut out -- in a procedure called a structured demolition -- so that investigators may conduct their work, police Agent Rich Bullerjahn said.

Officials declined to reveal the nature of the evidence that leads them to suspect arson until the investigation is complete.

Officials are keeping an eye on the building, Bullerjahn said. There is "a slim possibility it could collapse," he said.

The 300 block of University Avenue will remain closed for at least three days to all auto traffic and to pedestrian traffic on the Walgreens' side. The 500 block of Bryant Street will also remain closed.

Both the 400 block of Bryant and the 200 block of University will be closed to vehicle traffic.

Other stores in the 300 block will be closed indefinitely due to electricity being cut off -- including Subway, Gleim Jewelers, Sports Memorabilia and Satura Cakes. The stores could reopen if they can secure electricity independently, according to Bullerjahn.

Both Satura Cakes and Sports Memorabilia sustained damage from standing water in their basements, but not from fire, Bullerjahn said.

People with prescriptions at Walgreens may get them filled at any Walgreens, according to Bullerjahn, acting as public information officer at the scene.

Firefighters are still extinguishing hotspots in the building, although no smoke is visible from the two-story structure.

There will be 24-hour security at the Walgreens, Bullerjahn said.

Earlier today, an arson investigation team canvassed local businesses, asking if any suspicious persons had been spotted recently in the area, according to one store owner.

The Palo Alto Police Department, the Santa Clara County Arson Investigation Task Force and the federal ATF (Alcohol, Tobacco and Firearms) have been brought in to investigate the origins of the fire.

Palo Alto officials called in a structural engineer at 1:30 p.m. today to determine if the support columns in the Walgreens building were strong enough for the building to remain standing.

No precise age of the building has been determined but it is at least 70 years old according to historical photos.

The fire in the offices above Walgreens started at 9:35 p.m. Sunday night and burned for more than five hours. During the inferno, windows exploded, showering glass onto the sidewalks below. Flames shot more than 20 feet in the air, according to witnesses. More than 50 firefighters fought the blaze.

The last Palo Alto fire of this magnitude was at Town & Country Village about 10 years ago, Bullerjahn said. That fire burned for two days and closed down Hobee's restaurant and Jamba Juice for months.

Don Kazak, Becky Trout and Molly Tanenbaum contributed to this story.

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