In Pennsylvania, at least two people were killed and 10 others injured Monday as a U.S. Steel plant outside Pittsburgh exploded. Witnesses described a massive shock wave that sheared away part of a building, while billowing black smoke forced nearby residents to shelter in place for several hours due to fears over air quality. It’s not the first industrial accident at the plant. In 2009, a worker was killed in an explosion; one year later, 14 others were injured in yet another blast.