Thirteen people, including one gunman, have died after a shooting spree at the US Navy Yard in Washington, officials have said.
Police were searching the Navy complex to track down the gunmen involved in the attack on Monday morning.
The Washington police chief, Cathy Lanier, said one shooter had been killed and that two other suspects - a white and a black male - were still at large.
The dead suspect was identified by the FBI as Aaron Alexis of Fort Worth, Texas. Lanier told reporters that Alexis "was engaged in shooting with police officers" when he died.
"We have no indication of motive at this time," Lanier said.
Alexis, 34, received a general discharge from the Navy Reserve in 2011 after a series of misconduct issues, a Navy official said.
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Al Jazeera's Kimberly Halkett, reporting from the scene, said there was an "enormous police presence" in the vicinity of the Navy Yard after the attack. Two helicopters were flying over the complex and roads were blocked.
Thousands of employees were initially told to stay inside the complex while multiple SWAT teams searched for the shooters.
The Navy said the first shots were fired at 8:20am (13:20 GMT) in the headquarters building of the Naval Sea Systems Command, where about 3,000 people work.
"We heard some sounds ... we didn't know what it was, we had people running in the hallways, and we told everybody to evacuate, a couple of minutes later," Tim Jirus, a Navy commander who was in the building as shooting erupted, told Al Jazeera.
"The fire alarm inside the building went off, which was a way to get everybody outside the building."
Al Jazeera's Rosiland Jordan, in Washington, described the attack as "shocking", since the Navy complex is a high-security facility.
"It's a fortress," she said, adding that staff need to present ID cards to get in and that cars are checked to get access to the complex grounds.
The complex is located right next to residential buildings, businesses and schools, she added.
Todd Brundidge, an executive assistant with Navy Sea Systems Command, said he and other co-workers encountered a gunman in a hallway of their building on the third floor. The gunman was wearing all blue, he said.
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"He just turned and started firing," Brundidge said.
Flights at Reagan National Airport were temporarily halted to enable helicopters to transfer injured to hospital.
Janis Orlowski, a hospital spokeswoman at Medstar Washington Medical Center, said the hospital was treating two civilians and one police officer who were in critical condition with gunshot wounds.
President Barack Obama pledged to do everything possible to make sure whoever was responsible for the attack was held responsible.
He said the victims were "courageous Americans" who knew about the risks of serving overseas, but would not have expected such "unimaginable violence" at home.
Naval Sea Systems Command builds, buys and maintains the Navy's ships and submarines and their combat systems.
It is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget.
The Navy Yard complex is located 1.6km south of the US Capitol. It includes a museum and the residence of the chief of naval operations, among other buildings.
Source: Al Jazeera and agencies