Four people were killed and 12 were shot - two of the victims police officers - at the Washington, D.C. Navy Yard Monday morning by more than one gunman.

At about 10:20 a.m., one shooter had been reported contained. However, at least one other gunman remained at large.

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 said the shooting began at about 8:20 a.m. with three shots fired 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.

Read it at NBC Washington.