Philadelphia officials were facing tough questions over whether the accident could have been prevented.
Philadelphia Mayor Michael Nutter and the city's commissioner of licenses and inspections, Carlton Williams, have conceded that complaints about the working conditions at the demolition site were not followed up on.
City officials said that a routine inspection had found no violations at the property before demolition began. Williams said that inspectors had visited an adjoining property in May after complaints were lodged, but they found no violations and did not return to the Market Street site before Wednesday.
"No subsequent inspection occurred to indicate there was any unsafe conditions," Williams said. "We did not follow up and we are definitely looking into that."
Nutter promised a "wide-ranging investigation" into how and why the building collapsed.
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.
ABC News' Colleen Curry contributed to this report.