No one was injured in the predawn incident in which shots were fired into two windows at the sprawling Defense Department just across the Potomac River in suburban Virginia.
Steven Calvery, director of the civilian Pentagon Force Protection Agency, told reporters that authorities would have to re-evaluate their assessment if they find the incident was part of a larger plot. He said authorities did not yet have any suspects.
Calvery said a number of his officers reported hearing five to seven shots fired at about 4:55 a.m. EDT near the south parking lot of the Pentagon. The building, and roads leading to the property, were shut down as officers did an initial sweep of the area. They were reopened 45 minutes later.
An internal search of the building found fragments of two bullets still embedded in two windows - one on the third floor and one on the fourth. The bullets had shattered, but did not penetrate, the windows, Calvery said. The windows were part of offices that are being renovated and they were unoccupied at the time.
He said he didn't know what kind of weapon was used but that it was probably a high-velocity rifle.
Officials said it was the first incident of its kind since early March, when a gunman opened fire at a security checkpoint into the Pentagon in a point-blank attack that wounded two police officers.
The shooter, identified as John Patrick Bedell, 36, of Hollister, Calif., was shot by police and died hours after being admitted to a hospital in critical condition. Authorities had no motive for the shooting, but there had been signs that Bedell may have harbored resentment for the military and had doubts about the facts behind the Sept. 11 terrorist attacks.
State, federal and local agencies were cooperating in Tuesday's investigation in which authorities were studying surveillance video and questioning witnesses. Calvery said they were looking at whether the shooting might be related to Monday's discovery of bullet holes in windows at the National Museum of the Marine Corps. in Triangle, Va., some 30 miles south of the Pentagon. That shooting also is believed to have been with a rifle and possibly during early morning hours.
The Pentagon agency was working with the FBI, Arlington, Va. police who helped conduct the sweep of the area and Virginia State Police who closed part of nearby Interstate 395 in the search of evidence. A dozen officers at around 9 a.m. walked side-by-side in a line as they combed through a grassy area on the south side of the building.
Calvery said that in addition to his officers, investigators also were talking to some construction workers who also heard the shots.
They also are considering the possibility that the shooter had been standing on the roof of a nearby building.
A spokesperson for Prince William County Police, which is investigating the museum shooting, said Tuesday that authorities do not know if the shootings are related but they are coordinating their investigation with Pentagon police. A cleaning crew at the museum associated with the Quantico Marine Base called police Monday when they noticed the bullet holes in windows high up in a part of the building that faces Interstate 95.
Police believe the shots were fired at the museum late Saturday or early Sunday, when no one was inside. Investigators used a crane to inspect the damage Monday. Because of the height of the holes, police suspect the bullets were likely fired from a rifle, but they are still working to determine what caliber of bullet was used.
Several glass panels were hit, causing about $20,000 in damage. None of the museum's artifacts - including a harrier jet hanging near the damaged windows - were hit.
Associated Press writer Matthew Barakat in McLean, Va., contributed to this report.