The Associated Press

Photo courtesy of Boston Fire Department Warren Payne (left) and Paul Cahill.

BOSTON — Fire broke out inside a Boston restaurant, killing two firefighters and injuring 11 others who became disoriented in the dense smoke, officials said Thursday.

The fire began Wednesday evening in the apparently closed Tai-Ho Mandarin and Cantonese restaurant and spread to an adjoining building in a one-story row of yellow-brick storefronts, where at least three other stores were damaged.

The cause of the fire and the two firefighters' deaths remained under investigation Thursday, officials said.

"When they arrived, they had heavy fire in the Thai restaurant with heavy smoke conditions on the block," Fire Chief Kevin MacCurtain said. "They became trapped and they were disoriented and they couldn't find their way out."

Fire Department spokesman Scott Salman said a ball of fire may have been created after a three-ton air conditioning unit partially fell through the roof.

The firefighters who died didn't appear to have been hit by the falling AC unit, which was lifted out of the blackened building early Thursday. However a burst of flames can be caused when a fresh supply of oxygen reaches a confined space with superheated air, Salman said.

MacCurtain said officials believe the restaurant was closed when the fire broke out.

Killed in the blaze were Paul Cahill, 55, and Warren Payne, 53, part of the first unit to respond to fire, Salman said. Two other firefighters were under observation at a hospital, and eight were treated and then released overnight.

Several blocks from the fire, the Engine 30, Ladder 25 firehouse was mostly dark early Thursday, with a U.S. flag flying at half-staff in front of the building.

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The last death of a Boston firefighter in the line of duty was on March 16, 1999, Salman said.