Fire officials are backing away from the initial theory that a blast of superheated gases that killed two D.C. firefighters battling a blaze in a Fort Lincoln town house was caused by a "flashover" -- an explosion of gases that turns into flames.

Hours after firefighters were called to the inferno, officials blamed a flashover for killing one firefighter immediately and injuring four others, two of them critically. But a preliminary investigation has raised doubts about whether that type of explosion occurred because furniture and other items on the first floor did not burn, Fire Chief Donald Edwards said.

"There was some flame and I think that the people who were there, because of the sudden and tremendous influx of heat and some visible flames, thought that a flashover occurred," Edwards said. "It appears that there wasn't a flashover, but these guys were apparently in the path of some superheated gases. Those who survived it stated that because of the sudden envelopment of the heat with some flames, it was a flashover. At this point, we don't know what it was."

The accidental electrical fire broke out in the basement of the town house shortly after midnight. In less than an hour, firefighter Anthony Phillips was dead, and firefighters Louis J. Matthews and Joseph Morgan Jr. were critically injured. Lt. Charles Redding was treated for burns, and firefighter Stanley A. Taper was treated for smoke inhalation. Matthews died Monday. Ezra and Laverne Norton, who lived in the town house, escaped without injury.

"A flashover means that everything in the room, the whole area, is ignited at one time," said a source familiar with the investigation. "There was little evidence of burning inside the room. There was no fire in that room, just a lot of heat. If you look at the scene, the notion of the flashover is not supported."

Fire investigators continue to search for an answer.

"We just don't know," Edwards said. "My concern has been to honor the sacrifice [the firefighters] have made for the citizens of this city and to bury our fallen comrades. A committee of senior officials will do an investigation to see what exactly put these people in a room where these tremendous temperatures were experienced. It would be premature for me to give any explanation now."

What fire investigators do know is that when firefighters arrived at the scene at 12:15 a.m., the first floor of the Nortons' home was full of smoke and a fire was roaring in the basement.

Officials also know that all of the equipment used at the blaze worked properly and that communication was not a problem, Edwards said. An internal investigation into the death of Sgt. John Michael Carter in a 1997 grocery store fire cited a series of equipment and command breakdowns, including Carter's having a faulty radio.

"Equipment failure and inoperability was not an issue," Edwards said. "Everything worked. The building was the firefighters' greatest enemy."

Last Sunday, Engine Company 26 arrived first and entered the first floor through the front door with Engine Company 10. Smoke filled the living room, dining room and kitchen, but the firefighters did not see flames. The two companies proceeded down a hallway toward the rear of the house while Engine 17 positioned itself outside to extinguish the basement fire.

The Nortons had called 911 from a neighbor's home, but firefighters were following department procedures by searching for more occupants, Edwards said.

"Regardless of whether they knew the people were inside or outside, it's our job to ensure no one else is there," he said. "The only way to do that is to enter the building and extinguish the fire and verify if anybody is there."

Battalion Chief Damian Wilk, the incident commander, was stationed outside and apparently had not been advised that the building was three stories tall, a source familiar with the investigation said. Only two stories were visible from the front of the building, making it more difficult for Wilk to direct firefighters.

In the back of the building, Engine 17 firefighters turned on a hose to fight the basement fire. On the first floor, Phillips advanced a backup line to assist Morgan, Redding and Matthews, the crew of Engine 26.

"They were standing on top of the fire," a source said. "I'm not sure that they knew the fire was directly beneath them."

As the basement crew worked to put out the fire, "the gases of the heat rose and it caused even more heat," said another source close to the investigation. "That's why the guys who were in there said that it got so hot. The heat rose and the guys were in a cooker. The construction materials that were burning made it easier for the heat to rise. There was nowhere to go."

When the gases exploded, Phillips, Morgan, Redding and Matthews ducked, but Phillips was knocked unconscious from the force of the blast, officials said. The other firefighters were able to escape. Officials discovered there were four missing firefighters and sent eight inside to rescue them.

The kitchen floor had collapsed but none of the firefighters had fallen through to the basement. Phillips was found unconscious; Morgan and Matthews were carried from the home semiconscious; and Redding was alert, officials said.

"It was like they were standing inside a stove," a source familiar with the investigation said. "Heat and flames are all interrelated. The flames don't have to be introduced to produce the heat."

Officials have not taken statements from all of the participants, sources said. Edwards said that because he has been preoccupied with the funerals of the deceased firefighters, he has not yet listened to communications tapes.

One issue Edwards hopes to clarify is whether there was enough ventilation at the time of the explosion. While the three companies fought the fire from the inside, other crews should have torn open windows and opened the roof to expel the superheated gases, Edwards said.

"I'm sure that ventilation was occurring; I just don't know if there was enough," Edwards said. "The investigation into all of these things hasn't begun."

The investigation also is expected to reveal who gave the approval for the Engine 17 crew to turn on the hose and whether those firefighters were aware that the other companies were standing on top of the flames, a source close to the investigation said.

"That's what teamwork is about," the source said. "If they were cleared to turn on the hose, the companies on the first floor should have known that [so they could position themselves closer to an exit]. We don't know if they did."

Phillips, 30, died of asphyxiation as a result of inhaling superheated air, soot and smoke, fire officials said. Matthews, 29, died of thermal burns. Morgan remained in critical condition at Washington Hospital Center, where he underwent a second operation on Friday.

CAPTION: Dion Robertson, who is in Engine Company 26, and Tera Martinez comfort each other after the burial of Louis J. Matthews.