The construction of the Walgreens building at 300 University Ave. made it easy for Sunday's fire in downtown Palo Alto to spread and difficult for firefighters to attack it, Palo Alto Fire Chief Nick Marinaro said today.

The building had a two-foot space between the second-floor ceiling and the building's roof, which challenged firefighters trying to get at the flames while enabling the fire to spread laterally.

The first arriving crews did get hose lines into the second floor but were driven out by smoke and flames, Marinaro said. The decision was then made to fight the fire defensively, pouring water on flames where they could be seen and to protect neighboring buildings. It wasn't until the roof partially collapsed around midnight that firefighters could pour water directly onto the flames from above.

But the roof collapse created new problems, Marinaro said. It created voids where the roof had caved in and areas where fighters couldn't get at the flames. "Then it got its second wind," Marinaro said. The fire that had started at 9:30 p.m. Sunday ended up burning for much of the night, frustrating firefighters.

More than 50 fire personnel from Palo Alto, Mountain View, Menlo Park, Santa Clara County and Santa Clara fought the blaze. Some 18 trucks and engines responded to the four-alarm fire, the first in downtown Palo Alto since a pub burned in the early 1980s.

Additional trucks and engines from neighboring departments staffed Palo Alto fire houses while the blaze was being fought.

Arson investigators hope to get into the building Friday after it is structurally shored up. Collapse of the roof has made the walls of the building unstable.

Arson is suspected, Marinaro said, although there is no forensic evidence yet.

The first arriving engine crew saw flames already on the roof.

"If a fire gets going that quickly, arson is always a question," Marinaro said.