A fire that authorities say was set damaged a dressing-room area yesterday in the Strathmore Hall music center, a $100 million concert hall under construction in North Bethesda that has been beset by cost overruns in recent months.

Three fires were started inside the building late Sunday night or early Monday morning, said Pete Piringer, a spokesman for the Montgomery County Division of Fire and Rescue Services. Two fires caused "minimal damage," Piringer said, but the other caused extensive smoke damage in a hallway behind the center's main stage.

The damage could cost $500,000 or more to repair, Piringer said. But other county officials said the total could be far less than the fire department's estimate. They would not estimate the cost of the damage until a more thorough review is completed.

Officials said it is unclear whether the fire will delay the scheduled February 2005 opening of the county-owned concert hall, which has been billed as a world-class arts venue.

"We're optimistic that it won't be a significant issue," said Montgomery County Council member Nancy Floreen (D-At Large), who sits on the board of the nonprofit Strathmore Foundation.

"It's horrible," said Theresa A. Cameron, executive director of the Arts and Humanities Council of Montgomery County. "I think that we in the arts community will rally around helping Strathmore in any way we possibly can."

Piringer said investigators believe the fires were set because there were no other obvious sources of ignition.

The symphony hall has an undulating roof and six-story windows. It is about 75 percent complete, officials said.

Financial problems surfaced recently after project managers hired a consultant to make sure the building's acoustics were perfect for world-class musical acts such as the Baltimore Symphony Orchestra, which is scheduled for 30 performances there next year. The building's air-circulation system was moved for fear that the noise would disrupt performances.

In February, the County Council approved an additional $3.3 million to help cover $9.6 million in cost overruns. Some council members bridled at paying more money because of a vow the council made in 2000 to cap spending on Strathmore at $89 million.

No damage was visible yesterday from outside the building, and officials said the fire caused no structural damage. "It's basically cosmetic," said Oscar Garcia, another Fire and Rescue Services spokesman.

A construction worker called 911 about 4:30 a.m. yesterday to report smoke in the building, Piringer said. Investigators quickly determined the fires had been set, Piringer said.

Dennis Darling, senior vice president for Clark Construction Group Inc., the project's general contractor, said yesterday that the company was assessing the damage and could not say whether the building's opening will be delayed.

"At this point, we're still doing cleanup," Darling said. "Over the next day or so, we're going to go in and figure out what the best course of action is."

The damage should be covered by Clark's insurance, said Donna Bigler, a spokeswoman for County Executive Douglas M. Duncan (D).

"I'm quite confident it's not going to be an additional burden on the taxpayer," said council member Tom Perez (D-Silver Spring).

Damage was not visible from outside Strathmore Hall, under construction in North Bethesda, and officials said there was no structural damage.