Virginia State Police arson investigators said yesterday they have made a preliminary determination about the cause of the April 4 fire that destroyed Wolf Trap Farm Park's Filene Center, but will not reveal anything about their investigation until they complete at least a dozen more interviews.

"The less I say about this the better," said Robert E. Smale, the chief Northern Virginia arson investigator for the state police. Smale said he hopes the team of state police and U.S. Park Police investigators will announce the results of their probe next week.

Smale said that it took investigators more than two weeks to excavate the rubble that was once a 3,500-seat, cedar-and-steel amphitheater that Park Service officials say will cost at least $17.5 million to rebuild. The theater was built in 1971 for $2.3 million, which was given by philanthropist Catherine Filene Shouse.

Smale said investigators are interviewing people who "had something to do with Wolf Trap and might be able to tell us something about what happened." He declined to name those being interviewed, but a Fairfax County fire official said members of that department, including those who were on the scene the night of the blaze, are among those being questioned.

"We're doing the best we can, but it takes time," said Smale, who declined to say whether arson had been ruled out. The day after the blaze, fire officials said they believed it started near two wooden columns at the front edge of the stage. Smale said then that flammable substances including paints had been stored near the stage area.

Smale declined to disclose yesterday whether investigators had revised their theory about the origin of the fire.