HOUSTON -- A four-alarm fire that destroyed Gallery Furniture's north Houston warehouse and caused $20 million in damage has been ruled arson, investigators told KPRC Local 2 Thursday.

The Bureau of Alcohol, Tobacco, Firearms and Explosives ruled out all accidental causes.

"We have determined that this fire is an incendiary fire, meaning that the fire was intentionally set," National Response Team Special Agent In Charge Robert Elder said.

More than 120 Houston firefighters fought the blaze at 6006 North Freeway at about 8:20 p.m. on May 21.

"Employees tried to fight the fire, but it got away from them. They made a valiant effort to try and stop this fire from spreading. It could have been twice as bad had it spread to the showroom. You have to commend the Houston Fire Department for the tremendous job they did," Elder said.

The fire gutted the warehouse, destroying it and the millions of dollars worth of furniture inside. The fire did not spread to the store's main showroom, but the sprinklers inside it went off and caused extensive damage.

Dozens of customers were inside the showroom when the fire started.

"We're very, very fortunate, when you look at the size of this fire, that we didn't have anyone injured or killed," Elder said.

Officials declined to comment on exactly where the fire started and how it was started.

Investigators said they are interviewing several witnesses to determine the motive for the fire and who set it.

"We're doing neighborhood canvasses and we've had people simply call us and offer us information," Elder said.

Gallery Furniture owner Jim "Mattress Mack" McIngvale and his wife, Linda, are cooperating with the ATF's investigation.

"I think he's stunned. He's shocked. Mr. McIngvale has been very cooperative. He has given us everything we've asked for. We certainly feel for his loss. He just wants the person responsible caught," Elder said. "He has been interviewed and he has been very cooperative -- he and his wife both -- from the moment we stepped on his property."

McIngvale fought back tears when talking about the news he received from arson investigators.

"Gallery Furniture will overcome this hardship and we will rebuild. We will rebuild the world's greatest furniture store at 6006 North Freeway. Make no doubt about that," he said.

He said he had to remain strong for his hundreds of employees.

"They're concerned about people destroying where they work and people taking bread off the table for themselves, their children, their wives and their husbands," McIngvale said.

Elder said until investigators know who started the blaze, no one is being eliminated as a suspect.

McIngvale released a statement Thursday morning after the ATF news conference.

"Linda and I have been informed that investigators have discovered evidence that arson played role in last Thursday night's fire at Gallery Furniture's I-45 warehouse," McIngvale wrote. "We are shocked and stunned and want the person who is responsible caught. Linda and I want to thank the thousands of fellow Houstonians and Texans for their outpouring of support and encouragement. Make no mistake: Gallery Furniture is strong, stable and excited about our future. We will overcome this hardship so we can continue serving our loyal customers."

A reward is not being offered for information about the fire.

Anyone with information is asked to call 888-ATF-FIRE.

"Someone out there knows what happened that night and we're going to find them," Elder said. "Obviously, they have a blatant disregard for human life if willing to set a fire in a commercial structure that's occupied."

McIngvale is among Houston's most successful entrepreneurs and one of its biggest philanthropists.