BOSTON -- A restaurant fire burned for an hour undetected by chefs and patrons before gases ignited inside a ceiling and claimed the lives of two Boston firefighters, officials said Thursday.

"The occupants sitting in the restaurant did not know that there was a fire burning overhead above the double ceiling. The restaurant workers in the kitchen didn't know the fire was burning," Boston Fire Department Chief Kevin MacCurtain said. "It wasn't until a portion of the fire dropped down into the kitchen that they found out that there was a fire and quickly evacuated, calling for 911."

MacCurtain announced the findings of a preliminary investigation into the fire at the Tai-Ho Mandarin and Cantonese restaurant on Centre Street that broke out at about 9 p.m. Wednesday.

"(Firefighters) ran a hose line into the building, thinking to themselves, 'This one is going to be easy. I can see a little bit of fire in the kitchen,' and they brought a line in. When they started to extinguish the fire something unexpected happened -- something that we haven't seen in this type of fire," MacCurtain said.

While the cause of the blaze remained under investigation, officials said that grease from a flue venting up through the kitchen roof accumulated in a crawl space. After the fire ignited, the toxic and flammable gases began to build up in between two layers of the ceiling.

"Very quickly and very suddenly, the entire ceiling -- fiber ceiling -- was blown down under force. The volume of fire was pushed down that had accumulated in the crawl space. It accumulated in the crawl space because as the fire was smoldering and burning, it was giving off toxic and flammable gases. Those gases found enough air to suddenly ignite and flash and push the ceiling down. That's when the firefighters were very quickly overcome, and scrambling to get out," MacCurtain said.

Two firefighters dispatched to the scene, Paul Cahill, 55, of Scituate, Mass., and Warren Payne, 53, of Canton, Mass., were killed. Ten other firefighters were injured in the blaze. One firefighter who sustained smoke inhalation remained hospitalized Thursday afternoon.

"Last night's tragedy has devastated the Boston fire family. We are rallying around each other. Right now, our focus is to take care of the families of Warren and Paul," Union President Ed Kelly said.

Officials have established a fund for the families of the fallen firefighters. Donations can be sent to the West Roxbury Fire Fund, care of the Boston Fire Credit Union, 60 Hallet St., Dorchester, Mass. 02124.

Payne, a 19-year veteran, is survived by two children, and Cahill, a 14-year veteran is survived by three children.