Officials confirm Charleston fire began in loading dock

Story Highlights

• ATF says Sofa Super Store fire began in loading dock area
• Officials did not reveal the cause of the fire that killed nine firefighters
• Investigation will be "systematic, deliberate and tenacious," ATF agent says
• Employee has said workers smoked in dock area

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CHARLESTON, South Carolina (CNN) -- The massive furniture store fire that killed nine Charleston, South Carolina, firefighters began in the loading dock area, federal officials confirmed Saturday.

Officials did not reveal the cause of the fire, saying only where it began. The fire at the Sofa Super Store in Charleston on Monday night is second only to September 11 for the most firefighters killed in one event, according to The Associated Press.

When asked by reporters whether the blaze was caused by someone smoking, officials said they are still investigating. The possibility had been raised by an employee who said workers took cigarette breaks in the dock. Interactive: Details about the fallen firefighters

The investigation will be "systematic, deliberate and tenacious," said Ken Chisholm, a special agent from the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The loading dock was a covered space between the store's showroom and the warehouse and was a storage area for old furniture before it was thrown out, the AP said.

More of the firefighters were laid to rest Saturday.

Hundreds of people prayed and sang as members of a church choir clapped tambourines at a service for firefighter James "Earl" Drayton.

"Since late Monday night there has been the most extraordinary outpouring of love," Mayor Joseph P. Riley told the congregation. "It makes us able to face the future when we have these sad and heartbreaking times."

On Friday, thousands of firefighters traveled from across the nation to mourn the lost firefighters.

Autopsies showed the firefighters died from smoke inhalation and extensive burns, Rae Wooten, Charleston County coroner, said Wednesday.

The roof collapsed, but that alone did kill the firefighters, she said. Still, it is impossible to know whether that collapse trapped them inside, Wooten said.

Police said Saturday they likely will turn the property back to the owner in about a week so the debris can be cleared.

Charleston Mayor Joe Riley said the funeral for one firefighter was held Friday; six more are planned Saturday.