Machinery crawled over the site of a burned-out condominium complex south of Slidell on Thursday, shoring up what remains of the building so investigators from local, state and federal agencies can safely try to determine the cause of an early Sunday fire.

Some residents said they heard an explosion before seeing flames, according to St. Tammany Fire Protection District No. 1, which responded to the 1 a.m. fire. The cause of the blaze remains under investigation.

The fire apparently started in one of the downstairs units, according to spokesman Chad Duffaut.

St. Tammany Parish Sheriff Jack Strain said investigators are looking for witnesses and any video footage that people may have taken of the fire with their cellphones.

The New Orleans Division of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives announced Wednesday that it was activating a National Response Team to assist in the investigation of the fire that caused more than $1.5 million in damage to the Act X condominium complex.

The fire trapped a dozen people on their second-floor balconies; some had to jump into the arms of rescuers.

Kevin Moran, a special agent with the ATF’s New Orleans Division, said the special response team was brought in at the request of local and state officials and will play a supporting role in the investigation.

The size of the building and the fact it is a commercial structure are among the reasons the team was brought in, he said.

The ATF maintains four such teams, which are able to provide additional resources, including experts in fire investigation, explosives, forensic chemistry and photo mapping.

The team arrived Wednesday and held a briefing with officials. Work began Thursday to stabilize the building, which was heavily damaged, with fire affecting 60 percent of the structure and water damaging the remainder of the 28-unit complex.

“We were very fortunate that no one lost their life,” Fire Chief Chris Kaufman said.

Follow Sara Pagones on Twitter, @spagonesadvocat.