A natural gas explosion in eastern Pennsylvania leveled two houses, spawned fires that burned for more than seven hours and prompted the evacuation of hundreds of people. At least six people were unaccounted for Thursday.

Among them are two people whose two-story row house in a downtown residential neighborhood blew up about 10:45 p.m. Wednesday, police Chief Roger MacClean said. The cause of the explosion was unclear.

The fires consumed an entire row of homes, fire Chief Robert Scheirer said. The blaze was put out early Thursday, delayed by the difficulty of digging through packed layers of snow and ice to a ruptured underground gas line that was feeding the flames, Scheirer said.

He predicted eight houses would be lost and another 16 damaged.

In all, 500 to 600 people were evacuated, including elderly residents of a high rise. Residents were allowed to return home early Thursday. They had been taken to a Jewish community center and an agricultural hall at the city's fairgrounds while emergency crews worked overnight.

The blast was so powerful that it sent a flat-screen computer monitor sailing into the back of Antonio Arroyo, whose house was on the opposite end of the row from the house that exploded.

"I thought we were under attack," he recalled from a shelter where some 250 people took refuge in the hours after the blast.

Arroyo and his wife, Jill, both 43, lost their home in the fire.

Antonio said he ran outside and saw that an entire house had been leveled, a fireball now raging in the spot where it once stood.

"What I saw, I couldn't believe," said Arroyo, a community volunteer.

He and his wife, a nurse, fled their home with only the clothes on their back. They planned to return at daylight to see what they could salvage. Jill Arroyo broke down sobbing when she recalled her son's athletic memorabilia _ likely lost in the blaze _ including DVDs of his high school football games.

"The DVDs are gone. All his trophies are gone. All gone," she sobbed as her husband comforted her.

Tricia Aleski, who lives a few blocks away, said the explosion jangled her nerves.

"I was reading a book in the living room and it felt like a giant kicked the house. It all shook. Everything shook," she said. "I checked the stove and everything, (to) make sure everything's off."

Jason Soke was watching college basketball when he heard and felt the explosion. It rattled his windows. He went to the third floor and looked out and saw flames and smoke.

"Your senses kind of get stunned," he said. "It puts you on edge."