In March 1954 four Puerto Rican nationalists wanting independence from US rule, shot 30 rounds from semi-automatic pistols from the balcony of the House of Representatives chamber in the United States Capitol. 

The nationalists unfurled a Puerto Rican flag and began shooting as Representatives were debating an immigration bill.

Five Representatives were wounded, one seriously, but all recovered.

The assailants were arrested, tried and convicted in federal court, and sentenced effectively to life imprisonment.

In 1978 and 1979, their sentences were commuted by President Jimmy Carter. All four returned to Puerto Rico.

Source: Wikipedia