Takoma Park fire officials discovered yesterday that the explosions and fires that led to the evacuation of hundreds of residents Thursday was caused by gasoline that had been mistakenly poured into a sewer pipe instead of a gasoline holding tank.

The cause of the blasts was discovered after employes of Sickler Service, a plumbing and heating company at 7071 Carroll Avenue, attempted to fill up their green-and-white trucks yesterday and found its pump was empty, even though 2,000 gallons of gasoline had been delivered to the firm about 10 a.m. Thursday.

Chief Roger McGary of the Takoma Park Volunteer Fire Department said that when the gasoline arrived, the truck driver mistook a sewer standpipe for the opening to an underground gasoline tank and pumped the gasoline into the sewer. Chief McGary said it was the gasoline truck driver's first delivery job.

After discovering the mistake, police and fire officials, as a precaution, blocked off the area immediately around Sickler Service yesterday afternoon and flushed the sewer line with a chemical to neutralize any gasoline that might still be in the area.

Sickler is a small family-owned business that sits on a hill just above the area where the 13 explosions and fires occurred. The small beige building stands a few doors away from Takoma Towers, a high-rise apartment building for senior citizens. That building was not evacuated during the explosions on Thursday.

Andrew Sickler, who owns the plumbing company with his brother, Ronald, said last night that a 56-year-old man who had been working at the company for six months directed the delivery man to the rear of the building where the two 1,000-gallon gasoline tanks are located.

"I'm numb," Sickler said. "I can't believe that the guy (the delivery man) would ever put it down there . . . It doesn't resemble a gas pipe, it's a four-inch sewer clean-out." Sickler said the gasoline pipe has a two-inch opening and a locked cap.

Sickler said that he was on business in Virginia yesterday morning when he learned during a phone call that his workers were unable to get fuel from the tanks for the trucks. Sickler said that when he returned to the shop he realized that the wrong opening in the ground had been used.

"That's when the fire marshal came around. Then everything clicked," Sickler said.

Fire and police investigators said the dumped gasoline traveled with water through the sanitary sewer line, allowing fumes to seep into the basements of homes through storm drain traps in its path. Officials said that the fires and explosions were ignited either by pilot lights in basement heating units, or electrical sparks. There were no serious injuries as a result of the fires, the first of which was reported shortly after the gasoline was delivered to the plumbing company.

Officials evacuated homes in a 12-square block area of Takoma Park and Chillum, and the Washington Gas Light Co. turned off gas heating service to 241 homes. By 10 p.m. Thursday, all of the residents, except for those who had been burned out, were allowed to return to their homes. By mid-afternoon yesterday, 25 gas company employees had finished a door-to-door trek to relight pilots and restore heat to homes.

Red Cross workers helped four displaced families find temporary homes, and provided them with food and clothing.

By last night, only one family remained in a motel, where the displaced families had spent Thursday night.

Several emergency shelters were set up in Montgomery and Prince George's counties following the evacuations.

"The community really came through last night, everyone found a home to stay in," said Mary Whitlock, spokesman for the Montgomery Red Cross.

In Prince George's, the Office of Emergency Preparedness arranged for 73 people to spend the night at county expense at the Hampshire Motor Inn on New Hampshire Avenue. OEP director Hal R. Silvers said two families who were at the motel had been displaced by fire, and the rest stayed because their homes had no heat, or because their windows had been broken out by fire officials checking for vapor leaks.

The residents of Takoma Park rallied around each other the past two days, and the town's municipal center became a dropoff point for donated food and clothing.

A resident of the fifth floor of the Franklin Apartments -- a subsidized project of predominantly elderly and handicapped residents -- deposited a bag of canned goods on a desk at the center.

"We had an entire cross-section of the neighborhood--blacks, whites, Hispanics, several families speaking other languages--all of them in need and all of them helping each other," said Richard Schnuer, assistant city administrator.

Washington Post special correspondent Jim Brady contributed to this story. graphics 1/photo: By Douglas Chevalier--TWP An apparently false report of another explosion yesterday brought emergency and fire crews back to Takoma Park, after Thursday's explosions and fires. Officials said a loud noise in the area of Walnut Street and Eastern Avenue NE brought the equipment back to the still-nervous area. graphics 2/photo: By Douglas Chevalier--TWP A Washington Suburban Sanitary Commission employe checks for gasoline fumes.