A four-alarm fire early Monday morning that gutted a gay nightclub in the 7th Ward is being investigated by the New Orleans Fire Department, the state Fire Marshal’s Office and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

As of Monday evening, authorities had not been able to enter what remains of Club Fusions to try to determine what caused the fire, which ripped through the three-story structure at A.P. Tureaud Avenue and Aubry Street about 3 a.m.

Nobody was injured during the blaze, which occurred on a night when the club was closed.

The fire left the building still smoking late Monday morning. Seventy-two firefighters labored for nearly two hours to bring the blaze under control, according to Fire Department spokesman Capt. Edwin Holmes.

Holmes said the riverbound side of A.P. Tureaud as well as nearby blocks of North Galvez and Aubry streets were closed because the building was so badly damaged that it may collapse.

“There has been some structural collapse. There are still some hot spots within the debris,” said Brant Thompson, a state deputy chief fire marshal. “It’s going to be a difficult fire scene to investigate, I will say that.”

Thompson said investigators plan to deploy a drone to fly over the site — the only way to safely gather information at this point.

Club Fusions owner Keith Hamilton said the fire had left him mourning the loss of the business he ran for 21 years. The club was a rare gathering spot for young LGBT people, he said.

“It’s just a sad day for me,” he said. “I don’t know what the kids are going to do now because it was the only dance club for them.”

Hamilton said he had no idea what could have caused the fire. Aside from occasional derogatory remarks he has heard on the street, he said, the club has generally had few problems with either neighbors or patrons.

“You have some of the neighbors that didn’t like the gay club in the community, but we had been there so long,” he said. “We never had a shooting; we never had a stabbing.”

The Fire Department invited both the state fire marshal and the ATF to assist in its investigation. Both agencies have jurisdiction to investigate large commercial fires, though neither investigates all such blazes.

On Aug. 6, the Fire Department responded to two separate residential fires nearby in the 7th Ward, at Marigny and North Derbigny streets and at Frenchman and North Derbigny streets, both of which were deemed suspicious.