A Pentagon policeman was stabbed to death and several people were injured during a violent clash outside the sprawling building before fellow officers shot and killed the suspect.
The incident took place at a bus station outside the main entrance to the Pentagon, home of the US Department of Defence, which was temporarily placed on lockdown.
Authorities released few details about Tuesday's incident but confirmed the policeman's death, with the Pentagon's Force Protection Agency (PFPA) announcing it "mourns the tragic loss of a Pentagon police officer".
Several law enforcement officials told The Associated Press the suspect was Austin William Lanz, 27, of Georgia, who had run at the officer and stabbed him in the neck.
Responding officers then shot and killed Lanz, the officials said.
Investigators were trying to determine a motive for the attack and were digging into Lanz’s background, including any potential history of mental illness or any reason he might want to target the Pentagon or police officers.
Lanz had enlisted in the US Marine Corps in October 2012 but was “administratively separated” less than a month later and never earned the title Marine, the Corps said in a statement.
A volley of gunshots was fired but it was not immediately clear by whom. The gunfire resulted in “several casualties,” PFPA Chief Woodrow Kusse said.
US Secretary of Defence Lloyd Austin ordered American flags at the Pentagon to be flown at half-mast to honour the fallen officer.
Mark Warner, Democratic senator from Virginia, said his “heart goes out to the officer's family and friends, as well as the entire Pentagon police force" and Fairfax County Police Chief Kevin Davis ordered all officers to wear formal mourning bands to honour the deceased officer.
The Pentagon Transit Centre is only steps from the Pentagon, which describes itself as the world's largest office building and boasts of more than 27 kilometres of corridors running through the five-sided facility.
Built during World War II, the Pentagon normally hosts more than 20,000 employees, both military and civilian.
Chief Kusse said that after the investigation concludes, "we'll take a look at any measures” to enhance security at the Metro station and the Pentagon complex.
Hijackers flew a plane into the Pentagon on September 11, 2001, killing 189 people.
The lockdown was lifted more than an hour after the shooting, though police officers armed with rifles continued to arrive outside the Metro station where the incident occurred.
While some tourists and residents roamed the streets of neighbouring Pentagon City, a heavy police presence complete with armoured vehicles surrounding the entire perimeter of the Pentagon complex restricted public access to the building.
Pentagon spokesman John Kirby told reporters that Mr Austin was at the White House when the incident occurred and was informed immediately by his staff, who kept him updated throughout the day.
