One official said the work was progressing faster than expected.
"We have made fantastic progress in this investigation, however, it still has to be a very systematic, deliberate investigation," said Earl Woodham, a spokesman for the federal Bureau of Alcohol, Tobacco, Firearms and Explosives. "The reason for the fantastic progress may be that we have between 40 and 60 investigators into the fire scene."
Nine men perished in the blaze Monday night at the Sofa Super Store, blowing out windows and collapsing the roof of the building in the nation's deadliest firefighting tragedy since Sept. 11.
Mr. Woodham said that once pieces of the roof were removed Wednesday night, investigators were able to quickly go through wreckage, "focusing on areas where we believe the fire may have started."
He did not specify where that might be, but fire officials have said the blaze began in trash outside the building and then spread into the ceiling.
At the store Thursday, a large crane lifted pieces of the crumpled gray metal roof from the wreckage. Dogs trained to find accelerants also were being used. City officials have said arson is not suspected as a cause.
Meanwhile Thursday, city officials prepared for tens of thousands of firefighters and mourners expected to attend a memorial service to be held in a city sports arena at 10 a.m. today. Workers at the North Charleston Coliseum draped black bunting over the lighted signs that surround the outside of the arena, which plays host to minor league hockey games, college basketball tournaments and concerts.
Hundreds of blue folding chairs lined the floor, and lights were being set up to illuminate the stage.
The arena seats about 9,000 people, and officials planned to broadcast the service to screens outside and inside an adjacent performing arts center that seats 2,200 and a nearby convention center that seats 8,000.
On Wednesday, the assistant fire chief who decided the furniture store was safe to enter told The Associated Press the blaze appeared to have started in trash next to the building, but that it spread into a space between the building's ceiling and roof before firefighters arrived.
"I'm thinking that fire was in that ceiling above us," Assistant Fire Chief Larry Garvin said. "But when I was in there, I did not know it and they did not know it. If I could back up time ... all nine guys would be alive."
Assistant Chief Garvin said he saw smoke wafting between ceiling tiles inside the store, but thought it had seeped in through cracks and air vents from the trash fire on a loading ramp.
"When I walked in that building it was crystal clear," he said. "There was no smoke in that building whatsoever."
Firefighters rescued one trapped store employee by punching through a hole in the wall of the building and pulling him through.
LOCAL MEMORIAL WALL
A memorial wall to honor the nine Charleston firefighters who died Monday is on display at Stephen D. Posey Funeral Home in North Augusta. The public is invited to visit the memorial wall and sign a memorial book, which the funeral home will send to fire companies in Charleston. The memorial will be available to the public at the funeral home, 1307 Georgia Ave., from 9 a.m. until 5 p.m. through July 9.
- From staff reports