Video surveillance from that morning shows he first went up to the fourth floor and entered a men's restroom there, carrying a backpack, according to court documents.
After creating initial carnage on the fourth floor, Alexis returned to the first floor, where he fired on a security guard and took his Beretta handgun, FBI officials have said. Alexis then worked his way back to the third and fourth floors, "moving without particular purpose" and gunning down even more victims in the hallway and in their offices, FBI Director James Comey recently told reporters.
Court documents say Naval Criminal Investigative personnel were the first to respond to the attack and engaged Alexis. Members of the D.C. Metropolitan Police Department's Emergency Response Team later arrived and also exchanged gunfire with Alexis, according to court documents. His body was later found on the third floor.