WASHINGTON – Video footage inside Building 197 at the Washington Navy Yard shows Aaron Alexis "calmly" walking down the hallways of four floors and stepping into offices, firing indiscriminately at people and reloading a shotgun with shells he had stuffed inside the pockets of his black cargo pants.

The footage, disclosed Thursday by FBI Director James B. Comey in a news briefing, makes clear the 34-year-old civilian contractor on Monday morning was not targeting any individuals but rather was intent on shooting as many people as possible, even shooting a security guard and grabbing his weapon to continue the killing.

Indeed, Comey said, the rampage went on for at least half an hour until police responding to the base on the capital city's waterfront finally exchanged gunfire with him and cornered him inside, shooting him after he ran out of ammunition.

Alexis was shooting people, Comey said, "in a way with no discernible pattern." He said the shooter was "calmly moving without any particular direction or purpose. It was not as if he was looking for any particular person or group."

The director, who took over just two weeks ago, said Alexis had been working there on a computer server refresher project and was handling tasks throughout the building, making him familiar with the layout of the most crowded facility on the base. He added that Alexis, despite earlier news reports, never sprayed sniper fire down on employees in an atrium plaza below, but instead for 30 minutes roamed the halls and offices firing at will.

He said Alexis drove to the base about 8 a.m. and parked in a parking deck. He exited the car holding a bag, crossed a street and then entered the building. He proceeded to the fourth floor and entered a bathroom, coming out with the Remington 870 Express shotgun but not the bag.

What authorities do not know, Comey said, is whether the weapon was in the bag all along or whether Alexis might have planted the weapon inside the bathroom. He added the shotgun had been "cut down" — both the wooden stock and the barrel had been shaved to give him easier handling. He had purchased the weapon just two days earlier.

Comey said the rampage ended only when Alexis was out of ammunition. "He was isolated and pinned down by the first responders," he said.

He said the FBI has definitely ruled out any collaborators. "None," he said. "No sign of that." But he noted that initially there had been "some confusion" because of reports of other people running toward the gunfire, perhaps in efforts to stop him. "There are indications this was a person with mental troubles," Comey added. "And we are trying now to understand that."