Four hundred employees of the Naval Sea Systems Command began reporting to work at Washington Navy Yard building 197 on Monday morning after extensive renovations. The building has been closed since the September 2013 massacre, in which 12 people were killed and four others were critically injured. It was the second-deadliest mass murder on a U.S. military base, behind only the Fort Hood shooting in November 2009.The gunman, Aaron Alexis, was fatally shot in the head by police. Alexis was a civilian contractor and former Navy reservist who suffered from mental health issues. The Naval Sea Systems Command commented that the building has a new layout and appearance following the renovations. A total of 2,700 workers were displaced by the incident. Workers will gradually move back in over the next several weeks.