(Updates with one gunman killed, adds details, hospital statement)
By Ian Simpson and Susan Heavey
WASHINGTON, Sept 16 (Reuters) - Up to three gunmen, at least two dressed in military-style clothing, killed several people and wounded at least four others in a shooting spree at the U.S. Navy Yard on Monday, officials said.
A suspected gunman was among the dead and Washington D.C. police said they were searching for two other possible shooters they described as one white man and one black man. Officials said not all the facts were immediately known about the shooting that began at about 8:36 a.m. EDT (1236 GMT).
A Navy official said earlier that four people had died and eight others were injured after at least one gunman opened fire at the heavily guarded site, where about 3,000 people work.
But other officials suggested caution over those numbers saying the situation was in flux. Washington police declined to give a number of fatalities, saying there were "multiple victims deceased inside" the Naval Sea Systems Command headquarters, just 2.6 miles (4 km) from the White House.
Among the injured were a District of Columbia police officer, who was in stable condition in surgery, and one other law enforcement officer, officials said.
"There was three gunshots straight in a row," said Patricia Ward, who works at the Navy Yard, describing how she first heard the gunfire while having breakfast at the headquarters building.
A few seconds later, Ward said she heard four more gunshots. Security guards rushed in and got people out as fast as they could - "'Run, run, run,'they told people," Ward told reporters.
Washington Hospital Center said it was treating three gunshot victims, who were in critical condition, and was expecting more to arrive but it was not clear how many.
The shooting hit the military establishment less than three weeks after U.S. Army psychiatrist Major Nidal Hasan was sentenced to death for murdering 13 people in 2009 at Fort Hood, Texas, where he gunned down unarmed soldiers in what he later called retaliation for U.S. wars in Muslim countries.
Dozens of police and emergency vehicles surrounded the complex in southeast Washington, which is about a mile (1.6 km) south of the U.S. Capitol, local media reported.
Helicopters circled the headquarters with some touching down on the building's roof, according to a live feed by Washington's WJLA TV.
The Federal Aviation Administration said it briefly suspended departures at Reagan National Airport. The District of Columbia Public Schools have put six schools and an administration building on lockdown as a precaution.
The White House urged citizens to follow instructions from authorities and first responders and said President Barack Obama had been briefed.
The Washington Navy Yard is the oldest shore facility of the U.S. Navy. A former shipyard and ordinance plant, the facility in southeast Washington is currently home to the Naval Sea Systems Command, Navy museum and other facilities.
The yard, less than 2 miles southeast of the Capitol building, is a favorite for ceremonies. The Chief of Naval Operations, Admiral Jonathan Greenert, formally welcomed his Chinese counterpart to the United States in a ceremony there last Thursday.
Pentagon spokesman George Little said Defense Secretary Chuck Hagel was receiving regular updates on the situation and was "obviously very concerned of reports that there has been loss of life." Little declined to provide additional details on the situation, saying it remained "fluid and ongoing." (Additional reporting by the Washington bureau; Editing by Grant McCool)