LAKE WORTH, Texas (CNN) -- A church blaze on Monday that investigators believe was deliberately set killed three firefighters who were trapped when the roof collapsed.

The fire ripped through the Precious Faith Temple in Lake Worth, a suburb of Fort Worth.

Two of the victims, Brian Collins, 35, and Phillip Dean, 29, were Fort Worth firemen who were volunteering for a suburban fire department on their day off. The third victim, 19-year-old Gary Sanders, was a fireman from nearby Sansom Park.

Three other firefighters who were injured had been on the roof. One was hospitalized in serious condition; the other two were in good or stable condition.

Lt. Kent Worley of the Fort Worth Fire Department said the fire started in the back of the church and spread to the attic. The cause of the fire was being investigated.

Witness Kevin Schreiber, who works across the street from the church, said: "At one point, it was billowing smoke, everything was covered -- the whole sky, and the flames kept moving toward the front of the building. The wind has been ferocious like this forever. The roof all of the sudden just buckled."

Crews attacked areas of rubble where the missing firefighters were believed to be and finally reached the bodies more than two hours after the collapse.

"Even though we are firefighters, we have to go on doing our business as best we can," Worley said. "We're all human. ... It's not going to be an easy process."

Dozens of firefighters from Lake Worth and nearby fire departments were called to the scene. Police blocked off streets around the church, and helicopters hovered overhead.

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