The gunman who slaughtered 12 people at the Washington Navy Yard stalked his victims like a "hunter", officials say, in the chilling first portrait of how the carnage unfolded.
FBI Director James Comey told reporters former Navy sailor Aaron Alexis had roamed the offices and corridors of the Navy Yard's 197 building, blasting victims at random with a sawn-off shotgun before being shot dead.
Alexis, 34, was killed in a gun battle with police after being cornered following Monday's bloody spree.
Comey's account of the rampage came after analysis of security camera footage taken inside the 197 building.
The FBI chief said that after driving into the naval installation in the heart of the US capital early on Monday, Alexis had headed into building 197 and gone into a fourth floor restroom, carrying a bag.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
He came out moments later holding a sawn-off Remington 870 shotgun and began shooting anyone who crossed his path, Comey said.
"He emerged from the bathroom a few minutes after 8am with the shotgun and almost immediately started to shoot folks on the fourth floor in a way with no discernible pattern," Comey told reporters.
"It appears to me that he was wandering the hall like hunting people to shoot."
After shooting people on the fourth and third floors, Alexis went to the ground floor, shot dead a security guard and seized the man's weapon - a semi-automatic Beretta pistol.
He then returned to the third and fourth floors and continued shooting from the shotgun until he was out of ammunition.
Alexis then began shooting with the slain security guard's gun.
"That continued until the first responders arrived and then our team arrived and cornered him and sustained exchanged fire with him and he was downed and obviously killed at the scene," Comey said.
US officials have acknowledged missing "red flags" about the deeply troubled Alexis that may have averted the rampage.
It has emerged that Alexis had a security clearance that granted him access to the Navy Yard despite a record of misconduct in the Navy and run-ins with the law, including two shooting incidents and a Rhode Island police report showing he suffered from severe delusions.
