The Washington Navy Yard building that was the site of a mass shooting in 2013 has reopened after extensive renovations.

About 400 employees of the Naval Sea Systems Command began reporting to work at Building 197 on Monday morning.

The building has been closed since the September 2013 shooting, in which 12 people were killed and four others were injured. The gunman, Aaron Alexis, was killed by police. Alexis was a civilian contractor and former Navy reservist who suffered from mental health problems.

The Naval Sea Systems Command says the building has a new layout and appearance following the renovations, which also repaired damage from the shooting.

A total of 2,700 workers were displaced by the shooting. Workers will move back in gradually over several weeks.