Gunmen have killed four people and wounded up to 10 more at a US Navy base in Washington DC, according to reports.
Amid confused reports of possibly one, two or three gunmen, the respected Washington Post newspaper said two shooters, including at least one in military clothing, were involved in the attack at the US Navy Yard base on the Potomac River.
As police rushed to the scene to surround the base, Ed Buclatin, the public affairs chief for the Navy Installations Command sent a tweet saying: "four killed and eight injured" at the yard.
The Washington Post said that police claimed at least one of the shooters was "down," but it was unclear whether that meant the suspect had been arrested or shot. One federal law officer said a gunman at the scene was dead.
Two witnesses told CNN affiliate station WJLA that they heard a fire alarm go off in the building where they worked, then saw a man with a rifle down the hallway as they exited the building.
"He aimed the gun and fired our way," said Todd Brundidge said, adding, "I couldn't believe it." People frantically tried to run out of the building, Mr Brundidge said.
"Everyone was going down the stairs. They were pushing. They were shoving. People were falling down," he told WJLA. "As we came outside, people were climbing the wall trying to get over the wall to get out of the spaces. It was just crazy."
The navy said shots were fired in the building where about 3,000 people work. Staff were ordered to take cover where they stood. Streets in the area were closed.
The building houses Naval Sea Systems Command, the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget. It builds, buys and maintains the Navy's ships and submarines and their combat systems.
The White House said president Barack Obama was in Washington and aware of the shootings.