ALAMEDA — An old military dental clinic, abandoned and vacant for more than 30 years and known on the Internet as a haunted hospital, was destroyed in a two-alarm fire today, officials said.

After responding to two small "nuisance" fires at the site Saturday night, firefighters answered a call around 2:30 a.m. Sunday, though they could not enter the building.

"The city deemed the structure of the building unsound years ago," Acting City Manager David Brandt said at the scene this afternoon, as two fire engines and two fire trucks shot water into the building from a safe distance. "So they can't go in without doing some demolition first to open the building up."

The three-story clinic was set for demolition as soon as this summer, Brandt said. It is on the planned site of the Alameda Landing project.

Investigators would not be able to determine the fire's cause, or where in the building it started, until the fire was controlled and the building torn down enough to make it safe to enter, fire Chief Rick Zombeck said.

"We can't keep the kids out of it," Brandt said. "We have security patrol the area, and we put an eight-foot fence around the whole building, but kids see it on the Internet and cut open the fence and go in there to party."

The smoke from the blaze was so thick that the Fire Department issued a health advisory for everyone within a mile east of the building, where wind was carrying the thick gray and brown cloud.

"We're asking people to stay indoors and keep their windows and doors closed, because of all the smoke in the air," Zombeck said.

Fire officials worked to secure two assisted-living care facilities in the area as well, he added.

Zombeck estimated that extinguishing the fire could take until early Monday morning.