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Navy Yard gunman was 'wandering halls, hunting people to shoot'

New evidence is emerging in the shooting that killed 12 in the Washington, D.C. Navy Yard. It wasn't until nearly half an hour after the carnage began, the FBI says, that the first officers to enter the building had the shooter in their sights. And the FBI now says that there is no evidence Aaron Alexis fired from the upper floors of the building down into the atrium below. NBC's Pete Williams reports.

As authorities learn more about the movements of gunman Aaron Alexis, a clearer picture and timeline of Monday's Washington Navy Yard shooting, in which 12 people lost their lives, is beginning to emerge. 

Officials say Alexis, 34, drove onto the base at about 8 a.m. on Monday and parked in a deck across from Building 197.

He then swiped himself into the building using a valid pass and entered carrying a bag. He went to the fourth floor, entered a bathroom and emerged without the bag, James Comey, director of the Federal Bureau of Investigation, told NBC News.

The FBI said it remains unclear whether the shotgun was in the bag, but the weapon had been modified at both ends: Both the stock and the barrel were sawed off a little, Comey said.

Alexis then began shooting at people on the fourth floor "in a way with no discernible pattern."

"It appears to me he was wandering the halls and hunting people to shoot," Comey said. 

From there, Alexis moved to the third floor, shooting anyone who crossed his destructive path. Descending to the lobby, authorities said Alexis then shot a security guard and took a Beretta semiautomatic pistol from the man.

NBC's Pete Williams reports the latest developments on the Navy Yard shooter.

The gunman then continued moving up and down through the building, focusing mainly on the third and fourth floors. Comey said Alexis had shotgun ammunition in a pocket located on the outside of his pants. When he ran out of ammunition, he began shooting with the handgun, Comey said.

And despite earlier reports to the contrary, there is no evidence that Alexis shot from a higher floor down into the atrium of the building, Comey said.

Investigators believe he walked around the hallways that opened onto the atrium, moving without particular direction or purpose. Comey said there was no sign Alexis was looking for any particular person or group.

Victims of different backgrounds from all over the building were found both in hallways and offices, Comey said, adding that there is no indication at this point that any of the victims were hit by police or friendly fire.

One of the victims was hit outside the building, in an adjacent alleyway, officials said, but the man was hit by a round that came from inside the building.

The deadly rampage continued for 30 minutes before first responders and an active shooter team were able to get to the scene and cornered Alexis. Comey said a sustained exchange of gunfire followed, in which Alexis was downed and killed.

Comey said Alexis had been working on a project in Building 197, so he was familiar with it.

He said Alexis was killed roughly a half hour after the shooting started.

"I think the shooting starts a few minutes after 8 (a.m.). I think the first armed responders get there a few minutes after 8:30," Comey said.

Officials cautioned that the seemingly long response time was simply a matter of police not knowing about the shooting until it was well underway.

Cathy Lanier, chief of police for D.C.'s Metropolitan Police Department, has said that just two minutes after receiving the first call at 8:21 a.m., two units armed with AR-15 semi-automatic rifles were dispatched to the scene. Within four or five minutes, Lanier said, five to seven units were going through the gates into the facility.

Two police officers, one from the U.S. Park Police and one from  the Metropolitan Police, corned Alexis while combing a room full of cubicles. Alexis shot at them, and they returned fire, according to Ian Glick, head of the Park Police officers' union.

Glick said Thursday he had spoken with the U.S. Park Police officer and noted that the officer was still grappling with the incident.

"There was carnage everywhere in this building. Absolute carnage," said Glick. "When officers have to go past people who are asking them for help because they still have yet to neutralize the threat, that weighs heavy on your mind."