The letter did not contain any specific threats against the lawmakers or the site, but the U.S. Capitol Police and the FBI are now investigating the matter.
It was unclear which lawmakers received the letters, but House aides confirmed they were all Democrats. The letter was postmarked Feb. 28, a week before the explosion in New York.
The New York Times is reporting that the "improvised explosive 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 New York City-based congressional districts, according to House sources. It's unclear whether Democratic Sens. Hillary Rodham Clinton and Charles Schumer received similar letters.
A House aide suggested many more members could have received the letter and suggested law enforcement officials do not know the full scope of how many missives were actually sent.