At least two shots fired by an unknown gunman or gunmen struck the Pentagon early Tuesday morning, Pentagon officials told NBC News.
According to the officials, the shots were fired at approximately 4:55 a.m. on the south side of the building. A Pentagon police officer heard a series of at least five potential shots fired in or near the south parking area of the Pentagon.
Pentagon Police locked down the south parking lot and the entrances to the building, but an initial sweep didn't find anything. Until about 5:40 a.m., it was impossible to enter or exit the building or the property known as the Pentagon reservation.
"We did take prompt action, immediately closing down the reservation -- vehicular access as well as access to the building -- pretty quickly when the report of shots fired was verified," said Steven Calvery, director of the civilian Pentagon Force Protection Agency, which is leading the investigation.
Arlington Police and officers with canines responded for a second sweep, but again found nothing.
At about 6:20 a.m., someone noticed damage windows. Fragments of two bullets, presumably fired from a high-powered rifle, were found embedded in windows on the third and fourth floors. The bullets had shattered but not penetrated the reinforced-glass windows, which were part of unoccupied offices undergoing renovations. The bullets could help investigators identify the weapon used, NBC News Pentagon correspondent Jim Miklaszewski said.
Pentagon Police and Virginia State Police shut down part of southbound I-395 along the southern end of the Pentagon later Tuesday morning to search for possible evidence. A dozen officers walked side-by-side in a line as they combed through a grassy area on the south side of the building. The southbound lanes reopened at about 9:30 a.m., and the northbound lanes were then closed. Police were looking for shell casings.
Work crews were to remove the entire windows and frames from the Pentagon as evidence.
The FBI, Joint Terrorism Task Force and the Justice Department have joined in the investigation.
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Investigators are interviewing the officer who heard the shots as well as construction workers who said they heard the shots. Authorities also are reviewing surveillance video.
Over the weekend, several rounds were fired at the National Museum of the Marine Corps and Heritage Center in Triangle, Va., about 30 miles south of the Pentagon. Several glass panels were struck, but none of the museum's artifacts was affected. It was not immediately known if the two incidents are connected, but Prince William County police, which is investigating the museum shooting, is coordinating its investigation with the Pentagon.
For now, this morning's shots are being investigated as a random, isolated incident. Authorities will re-evaluate their assessment if they find the incident is part of a larger plot, Calvery said.
The Pentagon recently implemented security changes at the Pentagon Metro Station following a March 4 shooting there. John Patrick Bedell, 36, of Hollister, Calif., approached a pre-screening checkpoint that evening, produced a semi-automatic handgun when asked for a pass and exchanged gunfire with law enforcement, authorities said. Two Pentagon Force Protection Agency officers were wounded but stopped Bedell, who later died at a local hospital.
