In the wake of the collapse, Nutter has announced that every active demolition site in Philadelphia was being inspected for safety. He also announced a series of new rules for demolishing buildings within his city, including requiring a prohibition on using demolition machinery on a building if it is next to an occupied structure and mandatory drug tests and background checks for those operating heavy equipment on demolition sites.
At least 20 people were caught in falling debris when the building collapsed Wednesday around 10:45 a.m. An outer wall of the building that was being demolished fell outward and onto the two-story thrift store, according to city officials.
Eyewitness Dan Gillis was just working across the street when the building collapsed.
"They've been working over there for about a week now," Gillis told ABC News affiliate WPVI-TV. "It was a 30-foot wall. They started pulling on a piece of steel and I seen the whole wall just waving back and forth, and as soon as they pulled that out, there was no stopping it."
Roofer Patrick Glynn said he had been watching workers take down the building over the past few weeks and he suspected a collapse was inevitable because of the way they were going about it.
"For weeks, they've been standing on the edge, knocking bricks off," he told The Associated Press. "You could just see it was ready to go at any time. I knew it was going to happen."
A 10-block stretch of Market Street, which runs through the city, was shut down for the rescue effort.
Williams said that the building's owners and the contractors had all their permits and paperwork in order and up to date, and the building had no prior code violations.
ABC News' Colleen Curry contributed to this report.