A weekend fire, probably set by vandals, has ended any hopes of renovating a historic 91-year-old theater on the Forest Glen campus of the Water Reed Army Medical Center, Montgomery County fire officials said yesterday.

"There's no way they can do anything but tear it down," said Capt. Ray Mulhall, a fire department spokesman. "It's structurally unsafe."

Preservationists campaigned for more than 20 years to restore the Odeon Theater, one of several vacant historic structures in the Army compound, about two miles north of the Silver Spring business district. But an early morning fire Saturday made a ruin of the three-story building.

Because the building had no electricity and nothing was stored there, investigators suspect that the fire was set, said Ben Smith, a hospital spokesman. He said that youths often gathered at night in the empty building and that vandals had set a minor fire last spring. The Odeon was valued at $2 million, but Smith said the loss was more than a matter of money.

"Historically, it's irreplaceable," Smith said.

The Greek revival-style theater was part of a women's boarding school, National Park Seminary, that thrived until the 1940s. The school's founder, John A. I. Cassedy, erected a dozen buildings in architectural styles representative of different periods and nations. Cassedy wanted the students to be able to sample the cultures of the world at a single location.

The Army acquired the property in 1942 as a rehabilitation center. The Odeon, which had been the site of concerts, plays, graduation ceremonies and occasional political debates at the school, then became part of an annex to the Walter Reed hospital. Until closing the building in 1979, the Army used it to show occasional movies and for occupational therapy.

Preservationists, led by a group known as Save Our Seminary, were awaiting results of a study by Montgomery County planners. They hoped to find new uses for the buildings and raise money for repairs.

"This is devastating news for the community," said Derick Berlage (D-Silver Spring), a Montgomery County Council member.

The fire did not spread to any neighboring buildings, including Carroll House, a halfway house for 38 once-homeless men.

"If it hadn't burned down, I don't think they ever would have torn it down," said Gordon Minott, who heads Carroll House. "We've got to keep some of our past to remind us of our past."

Preservationists say two other important buildings survive from the days of the school -- a former resort hotel that became part of the seminary and a building known as the Ballroom that is more than 100 feet long with a 60-foot ceiling.