The United States Navy is reporting that there's an "active shooter" at the Washington Navy Yard, inside the Naval Sea Systems Command Headquarters building, as of 9:03 a.m. Eastern Standard Time.

There has been one confirmed injury, and a shelter has been established for emergency personnel on the scene.

The Yard hosts the command staff, headquarters directorates, and the Program Executive Offices. It is responsible for designing, building, and buying ships, submarines and combat systems for the Navy.

9:41 a.m. update: According to WTTG, police say they're looking for a black man, 6'1", bald, wearing a dark top and black pants and carrying an "AR-15 assault rifle."

9:49 a.m. update: The Washington Post is reporting that two of the injuries are believed to be critical.

10:29 a.m. update: Defense official reports 4 fatalities, multiple shooters, according to NBC News.

10:50 a.m. update: Washington Post says three shooters, one of which is down.

11:00 a.m. update: The same Washington Post story updated to say that two shooters are down now.

12:17 p.m. update: According to the most recent press conference, streaming below, police are "still looking for a white male seen around 8:35 in khaki tan gear, a red beret, and possibly naval uniform at 8:35 a.m. with a handgun." They are also looking for "a black male, 50 years of age, in possession of a long gun, wearing olive drab colored possibly military-styled uniform."

1:05 p.m. update: At least six people have died from the shooting, according to CBS News. At least ten other people were injured.

2:07 p.m. update: The number of fatalities has risen to twelve, according to a second press conference held on Monday afternoon. The FBI is now officially leading the investigation of the incident. Authorities have not determined a motive for the shootings.

3:45 p.m. update: Multiple media outlets have identified 34-year-old Aaron Alexis from Texas as the deceased gunman in a shooting. Officials have yet to publicly confirm those reports.

4:27 p.m. update: According to the Associated Press, Alexis had a criminal record and a concealed carry weapon permit.

5:07 p.m. update: KXAS-TV posted the arrest report for Alexis' September 2010 arrest in Fort Worth, Texas, for allegedly shooting into a neighbor's apartment. The Fort Worth Star-Telegram reported that he was charged with discharging a firearm within a municipality. Alexis argued that the gun went off while he was cleaning it.

A Fort Worth restaurant owner, Nutpisit Suthamtewakul, told the Telegram Alexis lived with him for three years, and that he considered Alexis his best friend at the time.

"I don't think he’d do this," Suthamtewakul told the Telegram before learning Alexis died at the scene. "He has a gun, but I don't think he’s that stupid. He didn't seem aggressive to me."

7:13 p.m. update: CNN reported on Twitter that investigators have ruled out the possibility of another shooter connected to the attack.

Also, the Associated Press reported that an unidentified man was arrested at the White House after allegedly throwing firecrackers over the fence, causing the area near the building to be cleared.

Watch a report from NBC News, aired Monday, below.

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[Image via Tim Hogan on Twitter]