A hazmat investigation at the building that houses the National Postal Museum has shut down several blocks in the immediate area as well as around nearby Union Station. U.S. Capitol police, D.C. police and D.C. Fire/EMS crews are on the scene investigating an unknown suspicious substance found inside a bottle. One employee inside Postal Square, located at the intersection of North Capitol Street and Massachusetts Ave. NE, tells DCist he’s been asked to shelter in place — meaning the building has not been evacuated, but no one is being allowed to leave, either. We’ll update when we have more details. MORE: No illnesses have been reported, but the bottle apparently came with a note that raised suspicions. Mass. Ave. is currently closed from New Jersey to Columbus Ave., as is North Capitol from E to G Streets NE. 10:17 a.m.: DDOT now says Mass. Ave. and North Capitol streets have reopened.