Five women and one man have been confirmed dead in Wednesday morning's collapse of two buildings in a busy commercial strip of Center City Philadelphia, even as a rescue operation continues, Mayor Michael Nutter said late Wednesday night.

Minutes after the 11 p.m. news conference had ended, firefighters could be seen suddenly digging furiously in the rubble. Searchers subsequently found a woman buried under debris, an official said.

Deputy Fire Chief Robert Coyne said early Thursday that 61-year-old Myra Plekam was pulled from the rubble more than 12 hours after the collapse and that she was awake and talking to rescuers.

She was taken to a hospital in critical condition, he said.

Thirteen other people were injured when a four-story building under demolition at 2140 Market Street collapsed onto the Salvation Army thrift shop next door.

"Our thoughts and prayers go out to those who lost their lives, and their families," Nutter said, adding he hoped the survivors will recover both physically and mentally from the tragic event.

The bodies of all six were recovered in the store, Nutter said, adding that the search-and-rescue operation is expected to continue through the overnight hours into Thursday.

Fire Commissioner Lloyd Ayers said late Wednesday night that rescue teams had about 40 percent of the Salvation Army building remaining to search. Nutter reiterated that officials do not know how many people were in the store at the time of the collapse.

Two of the fatalities were a 35-year-old woman, and a man identified by the Inquirer as Borbor Davis, 68, who was last seen working inside the store's basement.

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The tragedy began with sounds of the buildings crashing down at 10:43 a.m., which one witness described as "a freight train," when demolition of a four-story structure attached to the Salvation Army thrift shop at 2140 Market Street went horribly wrong.

City officials said they have not yet determined what went wrong.

Twelve people were initially taken to city hospitals after firefighters arrived at 10:45 a.m. following the initial report of the collapse. A 66-year-old woman was rescued about two hours later. About the same time, shortly before 2 p.m., the 35-year-old woman who died was also pulled from the debris.

The building that was undergoing "active demolition" at 2140 Market Street had up-to-date permits for the work, said Licenses and Inspections Commissioner Carlton Williams. He said a Philly-based firm, Griffin-Campbell Construction, received permits for the work in February. The company was working for the property owner, identified as STB Investments of New York City.

'This obviously could have been so much worse," Nutter said. He defended Williams, the L&I commissioner, when Williams could not answer any additional questions about the demolition permits and if inspections had been made at the work site.

The mayor also said it was not unusual for demolition work to go on directly next to open commercial properties or residential homes.

"There are demolitions taking place on a daily basis," Nutter said. "So it's not unusual that there would be people in a store or building next to where a demolition is taking place.

Marc Newell was doing landscaping work on 22nd Street when he heard an "unbelievable rumble."

The noise "was like a freight train," Newall said, adding it was immediately clear that people were trapped inside.

"You could hear the moaning," he said.

Some witnesses to the collapse were visibly distraught.

"The whole building just collapsed and people were stuck in there," said one woman, who was taken from the scene sobbing.

Brian Mullins, 23, was unloading paint at the nearby Mutter Museum when he heard the building fall. He and his coworkers ran toward the building, and pulled six people from the rubble, he said. At least two others remained inside when rescue crews arrived, Mullins said.

One woman was laying under a fallen wall.

"I just wanted to get people out," he said.

He said he tried to keep the people inside the building calm amid chaos in the moments after the collapse.

"The dust was so thick you couldn't see in front of your face," he said.

The Associated Press contributed to this report.