VALLEJO -- Investigators have determined that a three-alarm blaze Thursday morning that caused an estimated $500,000 damage to a vacant former car dealership in Vallejo was intentionally set, a fire department spokesman said.

The fire was reported shortly after 7 a.m. in an abandoned building at 4301 Sonoma Blvd. that once housed the Bill Lang Pontiac-Cadillac dealership, Vallejo fire spokesman Bill Tweedy said.

Firefighters removed several homeless people who were squatting in an adjoining building and then began combating the blaze, which was controlled in just over an hour, Tweedy said. One firefighter suffered a knee injury, and was treated and released.

Tweedy said the fire was intentionally set in a 55-gallon barrel that squatters were using to provide warmth, and also to burn the insulating covers off copper wire so that they could sell the product. The blaze then spread to the structure.

"It was intentional because someone did light the fire in the barrel, the fire was intentionally set," Tweedy said. "I doubt if they wanted to burn the building down, but that's what happened."

Tweedy said buildings on the property have been broken into numerous times by squatters.

"They basically converted the sales offices into condos for themselves," he said.

After controlling the blaze, firefighters remained at the scene for about two hours performing overhaul and conducting their investigation.