BREAKING: Four Dead, 12 Shot at D.C. Navy Yard; Active Shooter Still At Large; UPDATE: 12 dead in Navy Yard shooting; 1 suspect dead, 1 sought

(NBC) Four people were killed and 12 were shot - two of the victims police officers - at the Washington, D.C. Navy Yard Monday morning, and the shooter has not yet been apprehended, authorities confirmed.

A senior Naval official confirmed the number of victims to NBC News.

One of those shot was a D.C. police officer, reported NBC News' Pete Williams. Another was a base security officer.

The conditions of the victims are unknown, but George Washington Hospital reported that at least one victim has been transported to that hospital.

The U.S. Navy says three shots were fired at 8:20 a.m. inside Building #197 at the Naval Sea Systems Command Headquarters, 1336 Isaac Hull Ave. in the Southeast section of the District. U.S. military officials say the gunman - armed with an AR-15, a military-style assault weapon - first shot the base security officer.

One victim was reported to be on the roof of a building, reported News4's Tony Tull. A U.S. Park Police helicopter hovering above the scene around 9:50 a.m. lowered a basket to a building and lifted what appeared to be a shooting victim from the roof. The helicopter came back to the scene just after 10 a.m. to retrieve another victim.

But other details were sketchy from the still-active scene. The shots continued to ring out through the morning, including a volley that Tull reported at about 9:20 a.m.

Authorities say this a lone gunman.

Federal and local emergency personnel converged on the scene, "and a shelter in place order has been issued for Navy Yard personnel," the Navy said. Among the responders was a federal SWAT unit.

The 11th Street Bridge and roads in the area are closed.

Metro has also reopened New Jersey Avenue entrance at the Navy Yard station.

The Navy Yard is located in southeast Washington on the Anacostia River. The Navy says 3,000 people work inside the building, including command staff, headquarters directorates and field activities. They coordinate the Naval Sea Systems Command, or NAVSEA, the largest of the Navy's five system commands.

UPDATE:

WASHINGTON — At least 12 people died Monday during a shooting at the Washington Navy Yard, officials said. One gunman died in the shooting, and police are seeking another possible shooter.

The Navy Yard was placed on lockdown after shots were fired inside a building on the base, the U.S. Navy said. At least 10 other people were injured in the shooting, including eight civilians, one Washington Metro police officer and one base officer, D.C. Metro Police said.

Washington D.C. Mayor Vincent Gray said the gunman, whose name has yet to be released, was shot during an exchange with the Metro Police officer.

"As far as we know, it's an isolated incident," Gray said. "We have no known motive at this stage."

D.C. Police Chief Cathy Lanier had said authorities were seeking two potential suspects: a white male, 40 to 50 years old, wearing a khaki military-style uniform and a beret, carrying a handgun or pistol; and a black male, also 40 to 50 years old, 5'10", 180 pounds, medium complexion with gray sideburns and wearing an olive military-style uniform, carrying "a long gun."

Police later said the white male had been identified and was not a suspect.