WASHINGTON (AFP) – One gunman has been killed and two others may still be at large after a shooting spree at the US Navy Yard in Washington that left several dead, police said.

At least one police officer was among the wounded as police and federal agents descended on the area in a massive show of force, with streets blocked off only blocks from the Capitol buildings that house the U.S. Congress.

“We have one shooter that we believe involved in this that is deceased,” Washington police chief Cathy Lanier told a press conference. “The big concern for us right now is that we potentially have two other shooters that we have not located at this point.”

Lanier said one suspect was a white male seen with a handgun and wearing khaki clothes that resembled a naval uniform.

The second suspect was a black male in his 50s who possibly had a “long gun” and was clad in olive-drab, military-style uniform, she said.

But, with conflicting reports about the incident swirling, Lanier said the details on the suspects were not confirmed as police searched for the two alleged suspects.

As for a death toll, she said: “I will not give a confirmed number at this time, but I’d say we have multiple victims inside that are deceased.”

She did not say how the gunman died and earlier media reports had said a shooter had allegedly barricaded himself in a room in a headquarters building.

It was unclear how one or more gunman could have penetrated the heavy security that surrounds the Navy Yard, which is located on the Anacostia River, less than two miles from the Capitol.

The police chief confirmed a Washington D.C. police officer was among those injured, and hospital officials said he was in critical condition with wounds to his legs.

One employee at the Navy Yard, Patrica Ward, said she had just paid for her breakfast at a cafeteria when shots rang out.

“I was waiting for my friend to pay for when we heard the gun shot. It was three gun shots straight in a row, ‘pow-pow-pow,'” she told reporters.

“Three seconds later it was ‘pow-pow-pow.’ So it was like a total of seven gun shots. And we just started running.”

The guard “told all of us to just run, to get away as fast as you can.”

She said employees do not have to pass through a metal detector when they enter the building.

Police blocked off intersections around the Navy Yard as military troops in uniform stood guard at street corners and patrol boats moved in near the site along the banks of the Anacostia river.

Crowds of onlookers stood on sidewalks watching the drama unfold and at a construction site near the Navy Yard.

“I heard faint shots off in the distance. A bunch of cops came by and five minutes after we were evacuated,” said James, a construction worker nearby.

The U.S. Navy on its Twitter account said “several” people had been injured, while police officers told local media up to 10 people had been wounded.

Several schools in the area were on lock-down as a precaution, authorities said, and flights out of the nearby Reagan National Airport were delayed.

The Navy said at least three shots were fired at 8:20 am (1320 GMT) in the headquarters building of the Naval Sea Systems Command.

About 3,000 people work at the headquarters, which is responsible for building and buying U.S. warships and combat systems.

The site, which includes a naval history museum, dates back to the early 1800s, starting out as a shipbuilding center.

The complex also has a residence which serves as the home of the four-star chief of the U.S. Navy, Admiral Jonathan Greenert.

The White House said President Barack Obama had been briefed “several times about the unfolding situation at the Washington Navy Yard” by top security and counter-terrorism officials.

Flights from Reagan were suspended for 90 minutes, and the Federal Aviation Administration said it expected “residual delays as the airport returns to normal operations.”