Law enforcement sources are providing a new sequence of events into Monday's deadly shooting rampage inside Building 197 at the Navy Yard complex.
Federal authorities are piecing together a new timeline of Monday's mass shooting at the Washington, D.C., Navy Yard as they learn more about the movements of gunman Aaron Alexis that day.
They say the first call for help came at 8:21 a.m. that morning, and it took officers another 30 minutes to find Alexis.
Investigators believe that Alexis was concealing a shotgun with a sawed-off barrel and stock when he entered the building, though they still don't know if that gun was in the bag he was carrying or concealed in his clothing. And they know that the people he shot and killed were mostly on the third and fourth floors.
In a tragic twist, one of Alexis' victims was standing outside Building 197, in an alley, when he was shot by a bullet that passed through a door, the FBI told NBC News' Pete Williams Thursday.
The gunman was shot by a team comprising U.S. Park Police and D.C.'s Metropolitan Police who found him in an area filled with cubicles, U.S. Park Police said.
The team had to clear each cubicle, one by one, until Alexis engaged them and they shot him.
In all, 12 victims were killed, eight were injured and Alexis was shot and killed in the shooting Monday morning, the deadliest single event in Washington in decades.
Williams reports Alexis did not speak during the shooting, contradicting previous reports.
The new information, provided to Williams Thursday and in a U.S. Park Police press conference, is slightly different from some of what was reported earlier this week. The gunman never shot down into the atrium of Building 197, investigators now believe. And they know more about his weapons, one of which was taken from a victim.
Here is the new timeline of events during Monday's shooting:
At about 8 a.m., Alexis drove onto base and parked in a garage next to Building 197, the headquarters of the Naval Sea Systems Command. He swiped himself into the building using a valid pass, and carrying his bag.
Alexis went to a bathroom on the fourth floor. When he left the bathroom, he started shooting.
Alexis shot his victims mostly on the fourth and third floors, shooting randomly and with no apparent pattern, federal investigators said. At one point, he went to the first floor and shot a security guard, taking the guard's Beretta handgun, which he didn't use until he ran out of shotgun shells he was storing inside his cargo pants.
The shooting continued for 30 minutes before police and other first responders encountered Alexis, hidden in a maze of cubicles. Authorities have still not said which of the mulitple law enforcement agents shot and killed the gunman, but have confirmed "friendly gunfire" did not hit any of the victims.
They continue to probe Alexis' motive as well. Williams reports the shotgun Alexis used had the words "better off this way" and "my ELF weapon" carved into it, though it's not clear what those sayings mean. They believe he was suffering from serious mental problems, including paranoia and a sleep disorder.
It has been reported Alexis' background check years ago was done by the same company that cleared Edward Snowden.
Meanwhile, the families of 12 men and women killed were briefed by the FBI today and assigned Navy officers to help them. The first memorial service for a victim was held on Thursday.
The survivors, witnesses and first responders continue to grapple with the chaos and tragedy that they experienced.
The U.S. Park Police Officer who was one of the officers who shot Alexis had only been on the job three years and three months, and had to pass by people asking for help so they could enter the building to get to Alexis, Glick said. "That weighs heavy on your mind."
The Navy Yard reopened for business Thursday after being closed since Monday, though Building 197 still is considered a crime scene.