A powerful blast that blew apart a three-story apartment building in Fairfax County, leaving hundreds temporarily homeless, probably was caused by an accumulation of natural gas in the building's laundry room, fire officials said yesterday.

Surprisingly, they said, no one was seriously injured in the explosion or the fire touched off by the blast Saturday night at the 700-unit Southern Manor Apartments complex, located in Hybla Valley just west of Richmond Highway.

The explosion sent residents scurrying for safety, and some said there was a several-minute delay between the time of the explosion and the spread of the fire through one section of the 30-unit building at 7250-54 Fairchild Dr. The first firefighters concentrated on rescuing residents trapped on balconies, fire officials said.

Three of the people in the building at the time of the explosion suffered minor injuries, including bruises and a broken ankle. They were taken to Mount Vernon Hospital where they were treated and released.

Only the building where the explosion occurred was seriously damaged, with one outer wall blown off. Fire damage was confined to the one building, but the blast shattered windows of nearby buildings and blew pictures off apartment walls a quarter-mile away.

Tom Julia, a spokesman for Washington Gas Light Co., said last night that "We're going to defer to [the] opinion" of the fire department on the cause of the incident. Julia added that while it is "very possible" that a gas accumulation caused the blast, "what we'd like to know is what caused that . . . . "

He said that he understood that fire officials were investigating a theory on the cause of the gas buildup and that the utility was waiting for them to develop it.

Fire officials gave a preliminary damage estimate of $600,000.

On Saturday night, fire officials sealed off the building where the explosion took place and said it would be weeks before the building would be habitable. Officials also cordoned off four nearby buildings -- for a total of 107 units -- which were relatively undamaged.

By late yesterday afternoon, however, officials were restoring power and gas, which had been turned off by utility officials Saturday night, to three of the five garden-type buildings, and they said that only two buildings would not be habitable last night.

Three units in the burned building were ripped out by a crane, leaving a gaping hole, so fire officials could safely investigate the cause of the fire.

An additional two units in the building were all but destroyed, and the building was temporarily condemned by fire officials pending an examination to determine whether it was safe.

Late yesterday, officials were still sifting through the rubble from that section of the building but said all those who lived there appeared to have been accounted for, and they did not believe anyone was trapped in the rubble.

The Red Cross and Southern Manor management provided food and clothing for the displaced residents. Barbara Fakoury, a spokeswoman for the Red Cross, said 32 families were put up in the nearby John Yancey Motel Saturday night, and those needing shelter would be housed there again last night at no cost.

Southern Manor officials refused to comment yesterday. An afternoon news conference was abruptly canceled.

James Mills, a 27-year-old Woodbridge man who has relatives living in the building where the explosion took place, said he went to the basement moments before the explosion to check out complaints of a gas smell.

In the laundry room, he said, he found a sharp gas odor, and he said he saw "pipes hanging from the wall, just ripped in half" and heard "a sizzling sound." He said he was climbing the stairs from the laundry room to call the fire department when he was thrown against the wall by the explosion.

"It just threw me into the wall. I heard the patios falling, and I could see fire in the basement, and I just started running," Mills said.

Fairfax fire officials concentrated their investigation yesterday on two gas meters, twisted gas piping and a gas clothes dryer pulled from the basement laundry room where Mills said he saw the broken pipes.

Fire marshals and officials from Washington Gas Light Co. tried to fit the gas meters and piping together, and examined the dryer pulled from the rubble by the crane.

The three-alarm blaze, which began about 7:15 Saturday night, was brought under control by 8:15 p.m. by 85 firefighters using 25 pieces of equipment, fire spokesman Lt. Leonard Murry said.