Probably a false alarm from chemicals (gas odorizers) added to a combustible gas to aid in human nose detection, and people overreacting. See the initial story : https://www.rollcall.com/news/congress/176693-2 Could be heating gas leak, acetyline gas leak, plastic sealant smell. Could also have been a wayward briefcase, package, suitcase. EDIT: "Officials say a fire alarm was activated inside the building which led to the evacuation"