When the shots rang out at around 6:40 p.m. on Thursday night, hundreds of employees at the Pentagon were ordered to go into "Code Red" -- the entire building locked down, with no one allowed to enter or leave.
At least three ambulances were seen at the location, and all parking lots at the massive Defense Department headquarters were closed off.
After about 45 minutes, people were allowed to leave the Pentagon building through entrances other than the one closest to the Metro station, though Metro trains bypassed the Pentagon station.
Service to the station resumed around 8 p.m., according to a statement by Metro officials, though parts of the facility remained closed as police continued to investigate the shooting.
The Pentagon Metro station stop is a couple of hundred feet from the famous five-sided building. People leaving the station who wish to enter the Pentagon must show ID to Pentagon police in order to get close to the building.
The Pentagon, just across the Potomac River from downtown Washington, is, by area, the world's largest office building. About 26,000 people work there, according to the Defense Department.
The story has been updated since its original posrting. ABC News' Jason Ryan, Steven Portnoy, Martha Raddatz and Jennifer Wlach contributed to this story. Additional information from The Associated Press.