 Complete the following phrase:  On September 16, 2013, the Washington Navy Yard shooting occurred when lone gunman Aaron Alexis fatally shot twelve 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 the Southeast quadrant of the city. The attack, which took place in the Navy Yards Building 197, began around 8:20 a.m. EDT and ended when Alexis was killed by police around 9:20 a.m. EDT. It was the second-deadliest mass murder on a U.S. military base, behind only the 