The gunman who slaughtered 12 people at the Washington Navy Yard stalked his victims as if he was "hunting" them, FBI director James Comey said.
He said former Navy sailor Aaron Alexis roamed the Navy Yard's 197 building, blasting victims at random with a sawn-off shotgun.
The 34-year-old was killed in a gun battle with police after being cornered on Monday.
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, Alexis headed into building 197 and went into a fourth floor restroom carrying a bag.
He came out moments later holding a sawn-off Remington 870 shotgun and began firing at anyone who crossed his path.
"It appears to me he was wandering the hall like he was hunting people to shoot," Comey said.
After shooting people on the fourth and third floors of the building, Alexis went down to the ground floor, shot dead a security guard and took his 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 killed security guard's gun.
"Then our team arrived and cornered him and he was downed and killed," Comey said.
Comey said Alexis appeared to be selecting victims at random.
"From the video ... he appears to be moving without particular direction or purpose. His movements don't appear as if he was looking for a particular person or a particular group," he added.