13 Dead in U.S. Navy facility shooting, including gunman

The dead gunman has been identified by the FBI as Aaron Alexis, a civilian contractor for the military from Texas.

"There is no reason" to think that the mass shooting, in which up to three gunmen may have participated, was a terrorist attack, Washington Mayor Vincent Gray told a press conference.

The chief of the District of Columbia Metropolitan Police, Cathy Lanier, earlier had said a shooter was among those killed, although authorities are still looking for at least one other man identified as black - like Alexis - about 40-50 years old, carrying "a long gun" and said to be dressed in an olive drab military uniform or military-style clothing.

Lanier said earlier that authorities had several reports suggesting that at least two men - one of them white - had been seen carrying firearms in the vicinity of the shootings, but police later located the white man in question and ruled him out as a suspect.

At present, the motive for the massacre, which began about 8:20 a.m., is not clear.

The 34-year-old Alexis died under circumstances that have not yet been clarified by the authorities but unconfirmed reports indicate he was shot dead in an exchange of gunfire with police.

Alexis' military record indicates that he spent four years as a full-time Navy reservist between May 2007 and January 2011, when he was licensed as a contractor, the Pentagon said.

Authorities have not yet said how many people were wounded in the massacre, although Mayor Gray said that it was "a dozen or more."

A police officer and two women are being treated at the Medstar Hospital in Washington for wounds they received during the shooting spree.

Hospital spokesperson Janis Orlowsky said that those three victims are out of mortal danger - although two are still being operated upon - and are expected to make a full recovery.

One of the women was shot in the head, Orlowsky said, but the bullet did not penetrate the skull and so she did not need surgery.

The Sea Systems Command HQ is located inside the Washington Navy Yard, in the eastern part of Washington D.C.

Around 3,000 people work in the building.

While the FBI took charge of the investigation, the zone around the Navy Yard has been cordoned off and large numbers of police, including SWAT teams and helicopters, have been deployed in and around the area.

Security was strengthened at military buildings in the capital, including the Pentagon, as a "precautionary measure" since the situation at the Navy Yard is still not completely under control.

Operations were temporarily suspended at Washington's Reagan National Airport.

President Barack Obama received several briefings on the shooting from Assistant to the President for Homeland Security and Counterterrorism Lisa Monaco and Deputy Chief of Staff Alyssa Mastromanaco, the White House said.