At about 10:30 a.m. Wednesday morning, a four-story building collapsed onto a two-story structure in downtown Philadelphia, killing six people and injuring at least 13. Authorities say that at least a dozen were hospitalized with minor to moderate injuries. Rescue crews descended on the scene immediately after the collapse, digging through the rubble in search of survivors. The effort continued into the night. The Occupational Safety & Health Administration says it's investigating whether the buildings complied with safety standards, and early reports from the police indicate it may have been caused by an industrial accident from nearby construction crews.