Since you mention it:

In 1954, members of the Puerto Rican Nationalist Party shot 5 Congressmen during an immigration debate on the floor of the United States House of Representatives. - K @relevantmena 

• Jimmy Carter commuted their lifetime prison sentences in 1979. 
• Bill Clinton pardoned most of the perpetrators in 1999, but was rejected by Oscar Lopez Rivera. 
• Rivera received a second pardon from Barack Obama and a personal performance of “Hamilton” in his honor.
• In 1971 the Weather Underground detonated a bomb in a bathroom below the US Senate to protest American military operations in Laos. Nobody was ever arrested for this act. Bill Ayers, one of the ringleaders, went on to become a close friend and mentor to Barack Obama...