"Buildings get demolished all the time in the city of Philadelphia with active buildings right next to them. ... They're done safely in this city all the time," Mayor Michael Nutter said. "Something obviously went wrong here yesterday and possibly in the days leading up to it. That's what the investigation is for."
Authorities say they are confident no one else is trapped, and now the cleanup is under way.
Thirteen hours after the collapse of the four-story building, a 61-year-old woman was pulled out alive. The woman, identified as Myra Plekam of Kensington, Pa., remains hospitalized in critical condition.
"That's why we stay the course," City Fire Commissioner Lloyd Ayers said. "This person being pulled out alive is what this rescue operation is all about."
Plekam was one of at least 14 people injured in the collapse. Officials at the University of Pennsylvania hospital, which treated five victims, said that they saw mostly cuts and bruises on victims. One man and five women were among the dead, but their names have not been released.
It was unclear what role the demolition work might have played in the collapse, but the accident raised questions about how closely the highly visible spot in Philadelphia's Center City was being monitored. For weeks, people working nearby had watched with growing concern, as a crew took down the vacant building.
Roofer Patrick Glynn atop another building didn't think the operation looked safe.
"For weeks they've been standing on the edge, knocking bricks off," he said. "You could just see it was ready to go at any time. I knew it was going to happen."
And a pair of window washers across the street spotted an unbraced, 30-foot section of wall and predicted among themselves the whole building would simply fall down.
"We've been calling it for the past week - it's going to fall, it's going to fall," said Dan Gillis.
On Wednesday, that's what happened.
"Our thoughts and our prayers go out to those who lost their lives, and their families," Nutter said. "At the same time, we pray that those who survive will recover not only physically, but certainly mentally from the trauma of being in a building and it suddenly collapses."
Officials said the demolition contractor was Griffin Campbell Construction in Philadelphia. Records show that Campbell was charged in 2005 with dealing crack cocaine near a playground. The charges were dismissed after prosecutors misplaced evidence. He pleaded guilty in an insurance fraud case in 2009, and was acquitted of aggravated assault and related offenses in 2007.
Campbell, who was not available for comment, has also filed for bankruptcy protection twice since 2010.
There were no existing violations on the collapsed building, and Campbell had proper permits for the work being done, according to Carlton Williams, of the city's Department of Licenses and Inspections.
ABC News and The Associated Press contributed to this report.