When it was over, the suspected gunman lay dead amid an armload of weapons. Sources told News4 that surveillance footage showed that he began his attack with a shotgun, but was found with a 9mm pistol and an AR-15 assault rifle.
NBC News correspondent Pete Williams is reporting Alexis purchased a shotgun in Lorton, Va. during the past week or so.
READ MORE: D.C. Navy Yard Gunman Is Ex-Officer with Ft. Worth Ties The suspected gunman appeared to have seized firearms from two of his victims
as he moved through the building along the Anacostia River in southeast Washington, where 3,000 Navy employees go to work each day, many of them carrying authorized firearms.
Next to Alexis's body was an ID of a man who used to work at the Navy Yard. Some investigators believe the suspected gunman used that ID to enter the area, which is accessible only through guarded gates, but he may also have shot his way in, Williams reported.
The pandemonium continued after the suspected gunman was killed, as authorities suspected that more than one gunman remained on the loose. Eight nearby schools were put on lockdown. And as of Monday evening, D.C.'s mayor still said investigators are examining whether there was another gunman at large.
Terrie Durham, a civilian employee in the Office of Naval Sea Systems Command, was looking for a way out and glimpsed the gunman down a hallway, about 40 feet away. He was wearing blue, a uniform, maybe, and held a rifle. He said nothing. Then he fired.
"He aimed high and missed," Durham said. "I saw where the shot missed. It was a few feet ahead of us and a foot or above where we would have been standing."
She took off into a stairwell, hollering, "There is a man shooting in the building, get out, get out!"
Survellance video shows the gunman entered the NAVSEA building, at 1336 Isaac Hull Ave., with a shotgun
, law enforcement officials told News4's Jackie Bensen.
The suspected gunman shot a security officer in the head, killing him, and took the officer's 9 mm pistol and a magazine of ammunition
. The shooter then continued through the building and seemed to target his victims, who were mostly on the third and fourth floors, Bensen reported.
D.C.'s Metropolitan Police Department and several other law enforcement agencies responded with active shooter teams.
During that response, a MPD officer was shot in the leg. The gunman was then shot by a FBI hostage response team, law enforcement sources told Bensen.
“The response by uniformed police officers from both the Metropolitan Police Department and United States Park Police was absolutely nothing short of heroic,” Lanier said.
Authorities are investigating whether the gunman took the D.C. police officers' rifle.
WATCH: Raw Video: Police Respond to Shooting at Navy Yard
D.C. Police had said earlier Monday that they were seeking two other "possible" gunmen. Investigators later ruled one of them out as a suspect by watching video that tracked the man through the building and realizing that no harm resulted from his movements.Williams noted that many people are authorized to carry firearms at Navy Yard.
However, at an afternoon press conference, Mayor Vincent Gray said police were still working to rule out another suspect. Until that time, they are still seeking a "possible" second shooter, described as a man between 40 and 50 years old, who was wearing olive drab and was armed with a "long gun." He was about 5-feet-10-inches tall and 180 pounds, African-American, with a medium complexion and graying sideburns.
The motive is unknown, Gray said, though there were reports that Alexis may have had a grudge against the Navy. Gray said while there is no reason to suspect terrorism, it can't be ruled out yet.
The FBI now is leading the investigation into what is the deadliest single incident within D.C. borders since 1982, when Air Florida Flight 90 crashed, killing 78 people.