"We do not know and still do not know how many people were in the building," said Nutter, who noted that 13 people also were hurt with nonlife-threatening injuries.
"If there is anyone else in that building, they will find them," the mayor said of rescue workers. "They will stay here and do this work as long as it takes."
Nutter also stressed the need for privacy for the victims' families, a sentiment exemplified throughout the course of the search.
Shortly after 8 p.m., firefighters held up a yellow tarp to shield one of the bodies from public view as they carried the victim into a waiting ambulance. Over the course of the next couple hours, rescuers pulled several more body bags from the rubble as heavy excavation equipment stood outside the site, waiting for the grim recovery mission to end.
The late-night surge in the death toll also intensified the search for answers as to why a four-story building undergoing demolition suddenly tumbled in midmorning, cascading tons of wreckage onto a Salvation Army thrift shop.
Late last night, information began to dribble out about who may have died in the collapse, including a 35-year-old woman, the first confirmed fatality.
It appeared that one of the victims was the future daughter-in-law of Common Pleas Judge Robert P. Coleman. Last night, a man who did not identify himself came to the door of the judge's Roxborough home and said that family members could not talk of the tragedy.
"They're grieving," he said. "Thank you for your condolences." The judge's son, 27-year-old Bob Coleman, of Roxborough, wrote on his Facebook page that he worked at the Salvation Army and had just gotten engaged to the woman last month in Atlantic City.
Meanwhile, the Inquirer reported last night that a man who worked in basement of the Salvation Army store - Borbor Davis, 68, of Upper Darby - had died in the collapse.
It was a devastating end to what had started as a routine workday at the demolition site, as the steady drone of heavy machinery under bright crystal-blue June skies turned to tragedy in a rapid tumble of building material and debris.
The building collapse happened just before 10:45 a.m., forcing some passers-by to sprint for safety down city sidewalks, while others scrambled to pull dazed, bloodied survivors out of the confused jumble of a building that once housed the Hoagie City shop.
The sudden fall touched off a frantic daylong scramble through a dusty tableau of concrete and red bricks for both the living and the dead.
"I thought it was a terrorist attack because I'm originally from New York," Center City resident Ana Laufer said in the chaos immediately afterward.
At times, as many as 125 firefighters, paramedics and other emergency workers gingerly dug through the rubble to rescue victims, aided by a couple of rescue dogs.
At a 6 p.m. news conference, the mayor grew somewhat testy as reporters pressed him, city Licenses and Inspections Commissioner Carlton Williams, and others for information about city inspections and oversight of the building, its controversial landlord and their demolition contract.
"I'm not here for drama - I'm here to share some information," Nutter said as the journalists pressed for details. He noted that inspectors from federal agencies such as the Department of Labor and the Occupational Safety and Health Administration were already on site seeking to learn about the demolition and how the collapse occurred.
Authorities had no easy answers yesterday about what caused the collapse, other than to say it was an active demolition site. A power shovel was in the demolition zone, and its operator was among the five people injured who were treated at the Hospital at the University of Pennsylvania.
Elizabeth Datner, vice chairwoman for clinical operations in the department of emergency medicine at HUP, said five victims were treated there for minor injuries - all were in stable condition and felt lucky to be alive.
"We don't see buildings collapse every day," Datner said. "Everyone who came in was talking when they came and was able to speak to us and tell us how to contact their family."
"As you can imagine they were a bit stunned," she said. "They were saying that they heard a loud noise and the ceiling began falling."
Other victims were treated at Hahnemann University Hospital and Thomas Jefferson University Hospital.
The building is listed as owned by STB Investments of New York City, linked to Big Apple real-estate magnate Richard Basciano, who owns many blighted properties in Center City. For decades, a succession of administrations in City Hall have had complicated relationships with Basciano and with his mentor, the late Sam Rappaport.
Basciano and the controversial contractor hired to perform the demolition job will likely become the focus of the probe in the days ahead, but yesterday was mostly about the shock and awe of a building collapse on Philadelphia's main east-west thoroughfare.
The collapse wreaked havoc for both mass transit and for motorists. For hours after the collapse, police closed many streets in the normally bustling western portion of Center City to traffic, setting up a perimeter from 20th Street west to the Schuylkill and from Walnut Street north to the Ben Franklin Parkway.
"It's scary to see Market Street totally shut down - a street that normally is so busy," said Ed Davis, 42, who works at Jefferson Hospital. "It sounded like a random accident, but everyone first feared the worst."
Bryant Richardson, 37, who does building maintenance in the area, was in a meeting when he heard a TV news bulletin about the collapse. Richardson wondered whether a lapse of "preventive maintenance" prompted the shabby building to collapse.
Center City worker Carolyn Moore, 30, agreed: "For this to just happen out of the blue, this opens a lot of questions."
Patrick Kim, the trauma-program director at HUP, said it was fortunate that the patients they saw had fairly minor injuries - at least physically.
"We expect the physical injuries to heal up pretty nicely," Kim said. "I can't say too much about the other types of psychological trauma and other mental stress that they might be recovering from after something like this."
- Staff writers Solomon Leach, John Moritz, Stephanie Farr,
Morgan Zalot and Oscar Castillo
contributed to this report.
On Twitter: @DanaDiFilippo