A gunman kills twelve people at the Washington Navy Yard in Washington, D.C.

The Washington Navy Yard shooting occurred on September 16, 2013, when 34-year-old Aaron Alexis fatally shot 12 people and injured three others in a mass shooting at the headquarters of the Naval Sea Systems Command (NAVSEA), inside the Washington Navy Yard, in southeast Washington, D.C. The attack took place in the Navy Yard's Building 197; it began around 8:16 a.m. EDT and ended when police killed Alexis around 9:25 a.m. It is the deadliest mass shooting in Washington, D.C. history, as well as the second deadliest mass murder on a U.S. military base, behind the 2009 Fort Hood shooting.

September 16 is the 259th day of the year in the Gregorian calendar; 106 days remain until the end of the year.