(Updated: 2:45 p.m. ET)
Police have identified the shooter as 34-year-old Aaron Alexis, a civilian contractor from Fort Worth, Texas, using fingerprints.
Original story follows:
As many as 24 people were shot, including two police officers, and 12 killed at the Naval Sea Systems Command Headquarters at the Navy Yard in Washington, D.C., early Monday morning.
The U.S. Navy said three shots were fired around 8:20 a.m. at the headquarters, where about 3,000 people work.
At a press conference just after noon, D.C. Police Chief Cathy Lanier confirmed that one shooter involved in the incident was killed.
While 12 additional fatalities were confirmed, the number of dead was expected to grow as law enforcement officers sweep the buildings around the Navy Yard complex.
Investigators believe the shooter used the identification belonging to someone else to get into the secured Navy Yard Complex, and that person has been arrested and is being questioned by D.C. police to determine if he was involved.
Lanier said there were potentially two other shooters involved, with multiple reports they had been seen with weapons.
One was a white male, last seen at 8:35 a.m., wearing a khaki naval-style uniform and beret. He was seen on surveillance video apparently drawing a handgun, but it is unclear whether he was taking part in the attack or responding to the shooting.
The other was a black male, wearing olive drab military-style uniform with graying sideburns.
"We have no information to believe that either of those folks are military personnel, but we do have information that those individuals are wearing military-style uniforms," she said.
One of the police injured was a D.C. Metro policeman, and the other was a guard at the Navy Yard base, said Metropolitan Police Department Spokeswoman Saray Leon.
According to NBC News, the shooter was a black male carrying a long gun, possibly an AR-15, as well as a hand gun.
Three people were transported to MedStar Washington Hospital Center with multiple gunshot wounds, and all are expected to survive, said chief medical officer Janis Orlowski.
A "shelter in place" order was issued at 8:38 a.m. for all Navy Yard while police set up a perimeter, closing off several streets in the Capitol Hill area and shutting down one entrance to the Navy Yard Metro station.
Security was increased around Washington, including at the U.S. Capitol and schools in the area. Reagan National Airport remained opened to travelers, but the FAA said all flights had been halted because of helicopters near the airport's flightpath.
President Barack Obama has been kept apprised of the was briefed on the situation, and the White House released a statement:
"The President has been briefed several times about the unfolding situation at the Washington Navy Yard by Assistant to the President for Homeland Security and Counterterrorism Lisa Monaco and Deputy Chief of Staff Alyssa Mastromonaco. The President directed his team to stay in touch with our federal partners, including the Navy and FBI, as well as the local officials. We urge citizens to listen to the authorities and follow directions from the first responders on site."