A shooting rampage at a U.S. naval base in the heart of Washington claimed at least 13 lives Monday, including the gunman, while another possible suspect remained at large, police said.

The shooting sparked a massive show of force as police and federal agents surrounded the Navy Yard, cordoning off streets only blocks from the Capitol.

Officials gave no indication of any link to terrorism, while police said the motive for the attack on the installation was unknown.

PHOTOS: Muskets to M16s – All the Army’s Guns

Washington D.C police chief Cathy Lanier initially said there were two other potential shooters on the loose, but her deputies later said one of these suspects had been cleared.

Police were searching for a black male aged 40 to 50 clad in an olive-drab, military-style uniform, she said.

With streets blocked off, Lanier warned residents near the Navy Yard that police were still conducting an "active search."

"Stay in your homes and stay out of the area," she said.

As the FBI took charge of the investigation, conflicting reports swirled online, and a clear picture of exactly what had happened at the naval installation had yet to emerge.

Earlier media reports had said a shooter allegedly barricaded himself in a room in a headquarters building.

Defense officials would not confirm or deny reports that the shooter may have served in the Navy.

It was unclear how one or more attackers could have penetrated the heavy security that surrounds the Navy Yard, which is located on the Anacostia River, less than 2 miles (3 kilometers) from the Capitol.

The description of the other potential shooter wearing a uniform raised the possibility that the attack was carried out by insiders who had military passes to enter the facility.

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

People exit a building with their hands above their heads as police respond to the report of a shooting at the Navy Yard in Washington, on Sept. 16, 2013. Saul Loeb/AFP/Getty Images

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

"I was waiting for my friend to pay 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, as helicopters swarmed overhead.

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

Flights out of the nearby Reagan National Airport were briefly delayed and schools were on lockdown until anxious parents came to pick up their children in the afternoon.

The Senate adjourned for the day as a precaution and Washington's baseball team, the Nationals, whose stadium is adjacent to the Navy Yard, called off its Monday evening game.

The Navy said at least three shots were fired at 8:20 a.m. in the headquarters building of the Naval Sea Systems Command.

About 3,000 people work at the headquarters, which oversees the building and buying of 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 Navy, Admiral Jonathan Greenert.

President Barack Obama called the shooting a "cowardly act" and lamented that America was confronting "yet another mass shooting," saying troops in the military should not have to confront danger at home.