- NEW INFORMATION:
We now know 13 people died today after a shooting rampage at the Washington Navy Yard. They are all believed to be civilians or contractors. About half of the families have been notified as of this evening.
We've learned the shooter was discharged from the Navy for misconduct. He was working as a contractor for the Navy-- which answers the question of how he got in the Naval Yard.
But the FBI is still investigating what weapons he used, and how he got them in.
Police also said they are no longer looking for any other suspects in the shooting. They do not believe there is any further danger to the public from today's events.
A man brandishing an assault rifle, shotgun and handgun opened fire Monday inside a building at the Washington Navy Yard. The city police chief said 12 people were killed.
SWAT officers swarmed the building, the headquarters of the Naval Sea Systems Command, and a shooter there was killed, sources told NBC News. Law enforcement officials identified the gunman to NBC News as Aaron Alexis, 34, originally of Fort Worth, Texas. They said he recently began working as a civilian contractor.
Chief Cathy Lanier said at mid-afternoon that as many as two other suspects could be at large, both seen with firearms and wearing military-style uniforms. Minutes later, police said on Twitter that one of the two had been identified and was not a suspect or person of interest.
The rampage was the deadliest mass shooting in the United States since the massacre at Sandy Hook Elementary School in Newtown, Conn., last December, and the worst at a military installation since 13 people were killed at Fort Hood, Texas, in 2009.
The number of injured was not clear, but some reports placed it as high as 16.
President Barack Obama called it a "cowardly act." He said the rampage targeted patriots, military and civilian alike, "men and women who were going to work, doing their job, protecting all of us."
The Navy said the first shots were fired at 8:20 a.m. ET at the Sea Systems Command headquarters. Patricia Ward, who works at the Navy Yard, said she had just gotten breakfast in the cafeteria when she heard "three gunshots, pow-pow-pow, straight in a row."
"All of the people that were in the cafeteria, we all panicked, and we were trying to decide which way we were going to run out," she said. "I just ran."
The roughly 3,000 people who work there were told to stay in place. For the next several hours, conflicting reports circulated about whether the shooter was still alive, whether he had accomplices, how many places were active shooting scenes, and how many people were dead and injured.
In the meantime, chaos enveloped the surrounding neighborhood. Flights were briefly grounded at Reagan National Airport. Nearby schools and bridges the headquarters of the Department of Transportation were locked down. Farther away, police stepped up security on the Capitol grounds.
Many of the details were still uncertain at midafternoon. Hours after the shootings, the sergeant-at-arms of the Senate said he had recommended a "shelter-in-place" order to Senate leaders.
Terrie Durham, who works at the Naval Sea Systems Command building, said she saw a gunman who appeared to be wearing dark fatigues. Another worker there, Todd Brundidge, said he heard a fire alarm go off, and later saw the gunman come around the corner.
"He turned our way and started firing, and we ran downstairs to get out of the building," Brundidge said. "No words. He raised the gun and started firing."
A naval security guard was among those shot and was hit in both legs, U.S. military officials said. Washington city police told WRC, the NBC affiliate in Washington, that one of their officers was also among those shot. It was not clear how many of the others shot were civilian and how many were military.
Tim Hogan, a spokesman for Rep. Steven Horsford of Nevada, posted photos to his Twitter account of people tending to at least one person down on a street corner.
Washington police issued lookouts for two people they described as suspects — a 50-year-old black man with a rifle, wearing an olive drab military uniform, and a white man with a pistol, wearing a short-sleeved, tan uniform and a beret. They later said that the man in the tan outfit was not a suspect.
Dr. Janis Orlowski, the chief medical officer at MedStar Washington Hospital Center, said that the hospital was treating three gunshot victims — a woman hit in the head and hand, a woman hit in the shoulder and a man hit in the legs.
She said that the victims came in alert and talking.
"They're talking about gunshots that they heard in rapid succession," she said.
George Washington University Hospital said it had one patient.
Obama, speaking at an event marking the fifth anniversary of the financial crisis, turned to the shooting and said: "We are confronting yet another mass shooting, and today it happened on a military installation in our nation's capital."
"It's a shooting that targeted our military and civilian personnel," he said. "These are men and women who were going to work, doing their job, protecting all of us. They're patriots. And they know the dangers of serving abroad, but today they faced the unimaginable violence that they wouldn't have expected here at home."
The Naval Sea Systems Command builds, buys and maintains ships and submarines and their combat systems. The Navy Yard is along the Anacostia River in Washington, near the Washington Nationals baseball stadium. The Nationals postponed their game Monday night against the Atlanta Braves. The notation on the Nationals' website read, "Postponed: Tragedy."
Tracy Connor, Kasie Hunt, Michael O'Brien and Andrew Rafferty of NBC News contributed to this report.
Story By: Newswatch 12 News Team, NBC News and Associated Press