For the second time in 10 days, Montgomery County fire department officials evacuated a Silver Spring building yesterday, after occupants noticed a strong odor of gasoline.

The mystery fumes, believed to have come from gasoline being released into a storm drainage sewer by an unknown gas station owner trying to drain water from a leaky tank, forced approximately 50 people to leave the three-story office building at 8120 Fenton St.

Fire Department officials said the building would remain empty at least until 7 a.m. today, when state and county officals will examine underground gas tanks at nearby service stations.

"We're the only place in the whole damn area has gasoline," joked Polly Sessler, a cosmetician at Ecology Mart, next door to the evacuated building.

A fire department spokesman speculated that the fumes resulted from a service station owner's effort to remove water that had leaked into an underground tank. When the water is pumped out, he said, some gas may escape, too.

Service station operators in the area, contacted by telephone, denied that they had leaky tanks. One, Bobby Day, of Raley's Sunoco, said any loss of gas would show up in daily readings. He suggested that the problem stemmed from a leak in the neighborhood's natural gas main.

An official at Washington Gas Light Co. said natural gas was not the culprit. The utility sent a truck at the fire department's request and found a "gasoline concentration," and not a natural gas leak.

The utility official, Susan Butz, said natural gas was turned off so that pilot lights in the fume-filed building could be extinguished.