The nine Charleston firefighters who died Monday in the Sofa Super Store fire rushed into the type of structure experts say poses an extreme threat: a steel truss building.

Buildings with steel trusses can mask growing fires and cause roofs to collapse within minutes. Because of this, the federal government issues special safety guidelines and warns firefighters to get out if there is any indication that the trusses are exposed to fire. Federal workplace safety officials have long urged fire departments to use extreme caution when fighting fires in structures with trusses.

Though a fire may not be visible in the building, it can burn and spread through the roof system with little visible warning signs below, according to an April 2005 report by the National Institute for Occupational Safety and Health. "Fires in truss systems can burn for long periods before detection and can spread quickly across or through the trusses. Lives will continue to be lost unless fire departments make appropriate fundamental changes in fire-fighting tactics involving trusses."

Earl Woodham, spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives in the Carolinas, confirmed Friday that the roof over the store's center was supported by a steel truss.

The Charleston Fire Department did a pre-planning visit to the store in April 2006. Firefighters mapped the building's layout, noted that the building lacked sprinklers and that it contained furniture. They also recorded how many exits it had. The department's report does not indicate that the building had a steel truss design. One of the firefighters who did that inspection was the chief who was in charge when the firefighters went into the sofa store.

Buildings with truss systems are widely reviled in firefighting circles.

"There's no debate in the fire service," said Vincent Dunn, a nationally respected firefighting expert who has written textbooks on fire safety and survival. "We know it's a dangerous structure."

After Monday's fire, message boards and blogs were filled with firefighters lamenting the challenges of fighting fires in buildings with steel truss designs. Some talked about a saying in firefighting circles, "never trust a truss."

Monday's fire on Savannah Highway destroyed the 42,000-square-foot building and caused more than $2 million in damage.

Charleston Fire Chief Rusty Thomas said Friday that his department has fought plenty of fires in buildings with steel trusses. However, he said, "as far as if they knew it was steel truss construction and stuff like that, I don't know if my guys knew or not."

The federal report recommends that firefighters take extra precautions at fires involving steel trusses, such as opening concealed spaces quickly to determine where the fire is. But fires in buildings with truss designs often don't show signs they're about to collapse until it's too late.

Charleston Assistant Fire Chief Larry Garvin, who was one of the department's first commanders on the scene, said in interviews this week that he noticed a small amount of smoke near the store's ceiling when he first entered the building. He said the building was clear and that he made the decision to send firefighters inside in order to get a better angle on fighting the fire.

Carl Peterson is the director of the public fire protection division for the National Fire Protection Association, a private organization on whose recommendations many government fire safety rules are based. Peterson said truss systems can hide a growing fire above a structure's ceiling tiles and that firefighters should investigate any suspect areas before sending anyone into such places. "You need to confirm that if you are going to put people under it," he said. "If you are in doubt, you pull the ceiling and find out what's going on up there."

If a truss is exposed to fire, the report recommends that firefighters should immediately evacuate the building.

A lightweight truss system can fail within five to 10 minutes, Dunn said. "The National Fire Protection Association has been warning about steel trusses for 20 years."

Some of the nation's worst fire disasters have involved truss designs.

Since 2004, New York has required owners of commercial and industrial buildings with steel truss designs to mark them with a sign or symbol "that informs persons conducting fire control and other emergency operations of the existence of truss construction."

In addition to New York's state law, municipalities across the country also have instituted truss identification programs, often after deadly fires. South Carolina has no such requirement, according to the state fire marshal's office.

Dunn, the national fire expert, sums up what he has learned in his 42 years as a firefighter in two sentences in his book, "Safety and Survival on the Fireground":

"There are no new lessons to be learned from a firefighter's death or injury. The cause of a tragedy is usually an old lesson we have not learned or have forgotten along the way."