Here are a few more of the most prominent attacks (there were lots more):
On July 2, 1915, a former German professor at Harvard, Erich Muenter, planted a package containing three sticks of dynamite in the Capitol near the Senate Reception room. The explosive detonated around midnight and during a time when the Senate had been on recess.
On March 1, 1954, four Puerto Rican Americans fired guns in the House of Representatives, injuring five congressmen. President Jimmy Carter commuted one of their sentences in 1977, and granted clemency to the other three in 1979.
On March 1, 1971, a bomb exploded in the Capitol building. While the explosion did not injure anyone, it caused some $300,000 in damage. The Weather Underground claimed to be behind the bombing.
On October 18, 1983, Israel Rubinowits brought a home-made bomb into Congress. Her was deported.
On November 7, 1983, a bomb tore through the second floor of the Senate wing of the Capitol. The device detonated late in the evening and no one was harmed. The May 19th Communist Organization claimed responsibility. Susan Rosenberg was associated but not directly involved. Laura Whitehorn and Linda Evans were jailed but President Clinton commuted their sentences 16 years later.
In 1998, a man who claimed the U.S. was plagued by cannibalism and a fictional disease Caused the fatal shooting of two Capitol Police officers. He was institutionalized.