The letter did not contain any specific threats against the lawmakers or the site, but the U.S. Capitol Police and FBI are now investigating the matter.
It was unclear which lawmakers received the letters or when, but House aides confirmed they were all Democrats.
The New York Times is reporting that the "improvised exlposive device" used in the attack "was 'roughly similar”' to the devices used in two earlier bombings at foreign consulates in Manhattan in 2005 and 2007.
Update - The letters and pictures went to Democrats representing N.Y. City-based congressional districts, according to House sources. It's unclear if Democratic Sens. Hillary Clinton and Charles Schumer received similar letters.