Virginia arson investigators have tentatively decided that the April 4 fire that destroyed the amphitheater at Wolf Trap Farm Park was not deliberately set and that the blaze started in a utility and lighting control room beneath the right side of the outdoor stage.

"They told us that there is no physical evidence of arson at this time, but they haven't absolutely ruled it out because they haven't made a final determination," said National Park Service spokesman George Berklacy, who met with the five-member team of investigators yesterday.

Berklacy said the investigative team, composed of members of the Virginia State Police and U.S. Park Police, expects to interview about two dozen people during the next few weeks before issuing its final report.

The lighting control or dimmer room contained the sophisticated electrical equipment, including 100 dimmers, that supplied sound and light for the cedar-and-steel, 3,500-seat Filene Center amphitheater. The 20-by-30-foot room contained a temperature control unit to prevent the equipment from overheating.

Several days after the fire, Robert E. Smale, chief Northern Virginia arson investigator for the Virginia State Police, said flammable substances including paints had been stored near the stage area, which was under construction at the time of the blaze. It could not be determined yesterday whether those substances had been stored in the lighting room where the fire started.

Berklacy said the dimmer room was heavily damaged but not destroyed in the blaze. The theater was not equipped with an automatic sprinkler system or treated with flame retardant chemicals.

The Filene Center is named for the parents of philanthropist Catherine Filene Shouse, who in 1971 donated $2.3 million to build the theater. Last month Park Service officials estimated it would cost at least $17.5 million to rebuild the theater, and a fund-raising effort is under way.