A gunman opened fire on Thursday evening outside a Pentagon entrance before being severely wounded by police guards, authorities said.
The suspect was hospitalised with "pretty critical" injuries, said Richard Keevil, chief of the Pentagon Force Protection Agency.
Two Pentagon civilian police officers were hospitalised with what Keevil described as "grazing injuries", which were not life threatening.
The incident took place at about 6.40 p.m. when a man approached the sprawling office complex's busy entrance at the Washington Metro's Pentagon station. He was "very calm" and reached into a pocket for what the officers at the checkpoint expected would be a pass to enter the building but instead pulled out a handgun and shot the two officers, Keevil said.
"Both officers were able to return fire," he said.
The officers, armed with .40-calibre semiautomatic Glock handguns, wounded and disabled the suspect. "Many" shots were fired, Keevil said.
"We were fortunate that there was no other injuries," he said.
Keevil would not name the suspect but said he has been identified. In a press conference outside the Pentagon, Keevil refused to answer numerous questions about the suspect except to describe him as a US citizen "as far as I know".
Keevil said the officers at the scene described the suspect as showing no distress or nerves and not speaking before the shooting. "He walked up very cool," Keevil said.
Pentagon police were being assisted in the shooting investigation by FBI and Secret Service agents and sheriff's deputies from Arlington, Virginia, where the defence headquarters are located on the Potomac River across from Washington.
The Pentagon was locked down for at least an hour after the shooting, and the Pentagon subway station was closed, delaying trains during the later part of Thursday's evening rush hour.