(CNN) -- One person is dead and five others unaccounted for after an explosion believed caused by natural gas leveled several homes in Allentown, Pennsylvania, the city's fire chief said Thursday.
"We're actually looking for victims from the two homes that were leveled by the explosion and a victim from the third house, which suffered severe fire damage," Chief Robert Scheirer told CNN.
"I believe we're dealing with a recovery at this point," he said. "The fire is out. The gas line has been shut off. We have all the shelter victims being returned to their homes, the homes that can be habitable at this point."
The blast was reported just before 11 p.m. Wednesday, authorities said.
A pair of homes were flattened and burning after the initial explosion, with six more being consumed by the resulting fire, according to the fire chief. An additional 16 homes were affected by the blast.
The cause of the explosion wasn't immediately known, but Scheirer said utility crews came to the scene and shut off a gas line that was fueling the flames.
"We don't have anything confirmed yet at this point," Scheirer said when asked about the cause of the blast. "We believe it to be a natural gas explosion. We don't know if the leak was inside the home or out on the street. That all has to be investigated."
The blast occurred a short distance from the Gross Towers senior center, which was evacuated. About 500 people were in shelters immediately after the explosion. They were returned to the center Thursday, Scheirer said.
Firefighters' efforts were hindered by wintry weather, Scheirer told CNN.
"It's very cold. I was very concerned with my firefighters getting frostbite last night. We set up tents and stuff to try to keep them warm. We were relieving them as rapidly as we could. (It's) very difficult with the amount of snow that we've had here in the city lately .... And of course, once we start throwing water, everything turns to ice."