The mentally disturbed lone gunman who murdered 12 civilians at the Washington Navy Yard, appeared to be stalking his victims as if her were hunting them, authorities say after reviewing video footage of the shooter.

Former Navy Petty Officer Aaron Alexis, 34, had roamed the corridors of the Navy Yard's building 197, shooting victims at random, before ending up killed in a gun battle with responding policemen, FBI Director James Comey said.

Recent analysis of the security camera footage inside the 197 building provides the most detailed chronology of the events that led to the massacre.

Alexis could be seen entering the building and heading into a fourth floor restroom with a bag. He came out seconds later with a sawed off Remington 870 shotgun, and thus the carnage began, with Alexis shooting at anyone who had the misfortune of crossing his path.

"He emerged from the bathroom a few minutes after 8:00 am with the shotgun and almost immediately started to shoot folks on the fourth floor in a way with no discernible pattern," Comey told the media.

Comey then adds a chilling detail: "It appears to me that he was wandering the hall like hunting people to shoot."

Alexis continued to the ground floor having finished with the third and fourth floors, and shot a security guard. He takes the slain man's weapon-a semi-automatic Beretta pistol, and returned to the third and fourth floors to resume the shooting spree.

He stopped only when there was no more ammunition.

He did not appear to have a fixed target, selecting victims at random, but carefully taking aim, as witnesses fearfully recall.

"From the video... he appears to be moving without particular direction or purpose, his movements don't appear, at least to me, as if he was looking for a particular person or a particular group," Comely says. The people who had been shot dead, and those injured and left alive had been civilians, and all from different backgrounds. The only thing they had in common then was that they were all frantically trying to scramble for cover.

The FBI has confirmed that Alexis acted alone; initially there had been confusion, as several "good people" came running to the scene carrying weapons-understandably so.

Alexis was cornered by responding officers and killed in an ensuing shootout, taking with him all the answers toward his possible motivation with the shootings, and what could have possibly pushed him to do it.