Photos: Worst mass shootings in U.S.

Police officers walk on a rooftop at the Washington Navy Yard on Monday, September 16, after a <a href='http://www.cnn.com/2013/09/16/us/dc-navy-yard-gunshots/index.html'>shooting rampage</a> in the nation's capital. At least 12 people and suspect Aaron Alexis were killed, according to authorities.

Police officers walk on a rooftop at the Washington Navy Yard on Monday, September 16, after a shooting rampage in the nation's capital. At least 12 people and suspect Aaron Alexis were killed, according to authorities.