The discovery of an envelope containing white powder prompted the evacuation of a floor at USA Today’s headquarters in McLean on Monday in what emergency officials later said was a non-threat. An employee opened the envelope before 9 a.m. on the eighth floor, according to Fairfax County Fire and Rescue spokesman Dan Schmidt. That worker and another were decontaminated and sent to Inova Fairfax as a precautionary measure, he said. Schmidt said the county’s hazardous materials crews determined the powder posed “no credible threat.”