Initial reports are that two gunmen opened fire this morning at the Washington Naval Yards in Washington D.C.
U.S. Navy has confirmed at least ten victims of shooting at the Naval Sea Systems Command Headquarters
One gunman described as being six-foot tall African-American and wearing all black
Special ATF team that dealt with the Boston bombings in April sent to shooting site
One victim is a Washington D.C. police officer and another is a security officer at the base
One victim was airlifted from the naval base
All of the people wounded are in critical condition, authorities said
Flights at D.C.'s Reagan airport were grounded - but have now resumed
President Obama to speak before noon
Terror visited Washington D.C. this morning as at least one gunman opened fire inside a building at the Washington Navy Yard killing four people and injuring at least 12.
Witnesses reported at one man described as an African-American male in his 50s dressed in military fatigues and armed with an AR-15 assault rifle opening fire upon entering the base at the Naval Sea System Command HQ.
Though it wasn't immediately clear how, according to a Defense Department official and federal law enforcement official who spoke on condition of anonymity because they were not authorized to discuss the matter publicly.
Two other officials have said police were looking into the possibility of a second and third shooter.
The attacks came three days after al-Qaeda used the 12th anniversary of 9/11 to call for strikes on America.
Hundreds of SWAT and FBI rapid response units descended on the nation's capital to deal with the situation which unfolded just before 8.30 a.m. this morning.
Initial reports from the scene are that one of the suspects walked up to the facility, opened fire and then ran inside the building.
People who were inside the building said the gunman  described as a tall, African-American male  wordlessly sprayed fire from an AR-15 assault rifle.
'We saw him hold the rifle, and we saw him aim it in our direction,' said one witness to Fox News.
Authorities say a base police officer also was seriously injured.
One victim was reported to have been shot on the roof of a building, reported News4's Tony Tull.
A U.S. Park Police helicopter lowered a basket to a building and lifted what appeared to be a shooting victim from the roof just before 10 a.m.
SWAT officers, military police, U.S. Capitol police and Washington city police all rushed to respond.
Witnesses described a gunman opening fire from the fourth floor, aiming down on people in the first-floor cafeteria. Others said a gunman fired at them in a third-floor hallway.
As witnesses emerged from the building, a helicopter hovered over the building, schools were on lockdown and airplanes at nearby Reagan National Airport were briefly grounded. Less than 2 miles away, security was beefed up at the Capitol, but officials said there was no known threat there.
The exact number of people killed and the conditions of those wounded was not immediately known. About 3,000 people work at the Naval Sea Systems Command headquarters, which builds, buys and maintains the Navy's ships and submarines and combat systems.
Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other co-workers encountered a gunman in a long hallway of their building on the third floor. The gunman was wearing all blue, he said.
"He just turned and started firing," Brundidge said.
Terrie Durham, an executive assistant with the same agency, said she also saw the gunman firing toward her and Brundridge.
"He aimed high and missed," she said. "He said nothing. As soon as I realized he was shooting, we just said, `Get out of the building.'"
Rick Mason, a program management analyst who is a civilian with the U.S. Navy, said a gunman was shooting from a fourth floor overlook in the hallway outside his office. He said the gunman was aiming down at people in the building's cafeteria on the first floor. Mason said he could hear the shots but could not see a gunman.
Shortly after the gunfire, Mason said someone on an overhead speaker told workers to seek shelter and later to head for the gates at the complex.
Patricia Ward, a logistics management specialist, said she was in the cafeteria and heard shots. They sounded like "pop, pop, pop," she said. After a few seconds, there were more shots.
"Everybody just panicked at first," she said. "It was just people running, running, running."
Ward said security officers started directing people out of the building with guns drawn.
Police and federal agents from multiple law enforcement agencies responded. Ambulances were parked outside, streets in the area were closed and departures from Reagan National Airport were temporarily halted for security reasons.
Among the wounded was a D.C. police officer, according to a law enforcement official who spoke on the condition of anonymity because the person was not authorized to discuss an ongoing investigation.
A U.S. Park Police helicopter hovered over the building and appeared to drop a basket with a person onto the roof.
Officials at MedStar Washington Hospital Center said two shooting victims had been brought there.
District of Columbia schools officials said six schools and one administrative building in the vicinity of the Navy Yard were placed on lockdown. The action was taken out an abundance of caution, schools spokeswoman Melissa Salmanowitz said.
Janis Orlowski, chief medical officer at George Washington Hospital said that they have taken three people who are seriously injured but have good chances of survival.
The doctor said that all victims are conscious and speaking. She said the first male victim has multiple gunshot wounds to the leg.
The second victim is female and has been shot in the shoulder. The third victim is female and has been shot in the head.
The hospital said that it is expecting to receive more of the injured.
Naval Sea Systems Command (NAVSEA) is the largest of the Navy's five system commands. With a fiscal year budget of nearly $30 billion, NAVSEA accounts for one quarter of the Navy's entire budget.
The Navy Yard is along the Anacostia River in Washington, near the headquarters of the Department of Transportation and the Washington Nationals baseball stadium.
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