The 1971 bombing was not the first or the last time that politically motivated violence struck the Capitol.
On July 2, 1915, in the midst of World War I, a German-born professor at Harvard named Erich Muenter planted dynamite under a switchboard near the Senate Reception Room. Shortly before midnight, it went off. No one was injured. Muenter later wrote to a Washington newspaper to explain that the bomb was "an exclamation point in my appeal for peace." After being arrested for trying to kill financier J.P. Morgan, Muenter committed suicide in jail.
A few years after the Weather Underground struck, a bomb planted by a group calling itself the Armed Resistance Unit exploded under a bench just outside the office of Sen. Robert Byrd (D-W.V.) That explosion, a bit before 11 p.m. on Nov. 7, 1983, was one of a series of attacks across Washington, D.C., that year. The bombing, meant to protest the military's actions in Grenada and Lebanon, didn't cause any injuries.
In 1954, a group of Puerto Rican separatists got onto the House floor and opened fire with handguns. Five members of the House were wounded in the attack, including Rep. Alvin Bentley (Mich.). He's in the photo below, being carried out by pages.
The page at left in that photo is named Bill Goodwin. He later described what he saw.
Another photo available at the House historian's Web site shows a bullet-pocked desk, where Rep. Charles Halleck (Ind.) was sitting during the attack.
"I don't regret setting bombs," Bill Ayers said when interviewed by the New York Times on the occasion of the release of a book he'd written. Asked by the interviewer, Dinitia Smith, if he would do it over again, he blamed "the violence of the Vietnam War."
That interview, coincidentally, appeared in the paper the last time the Capitol was targeted for attack by a radical group, thankfully unsuccessfully: Sept. 11, 2001.