Police also confirmed today that a second individual taken into custody was only briefly detained and was released after authorities determined that he had nothing to do with the incident.
In response to a question, police said there was no need to change the police training procedures because the Fort Hood shooting last year "put us on notice this could even happen at a military reservation." He said police had the appropriate training and procedure in place and doesn't foresee a change "in how we do business."
Initially, 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 last night, 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. Thursday, according to a statement by Metro officials, though parts of the facility will remain closed as it is "a very complicated crime scene" with a lot of bullets, according to police.
Police are examining surveillance footage of the incident, which lasted under a minute, but said it will not be released any time soon.
This is not the first case of a fatal Pentagon shooting, Keevill said, adding that in the late 1980's a person with mental issues came on the reservation and shot a Naval officer, but "that was many years ago before 9/11."
The Pentagon Metro station stop is a couple of hundred feet from the famous five-sided building, which was hit by a passenger jet during the 9/11 terror attacks. 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.
ABC News' Jason Ryan, Sarah Netter, Steven Portnoy and Jennifer Wlach, and Reuters contributed to this story.