Washington (CNN) -- A Pentagon official said he believes the shooting early Tuesday at the U.S. Defense Department headquarters was a "random incident."
"We are looking at all the possibilities," Steven E. Calvery, director of the Pentagon Force Protection Agency, said at a news conference late Tuesday morning. "What we have is an isolated incident, so far."
Pentagon police officers, as well as several construction workers in the area, heard at least five shots fired around 4:50 a.m., Pentagon officials said.
As of midday Tuesday, authorities had discovered two bullet fragments in third- and fourth-floor windows on the south side of the building, said Calvery. That part of the Pentagon was empty at the time of the shooting, as it is in the process of being renovated. The bullets shattered but did not go through the windows, which are bulletproof, according to Calvery.
The incident prompted a 40-minute shutdown of the entire Pentagon, and authorities conducted an interior sweep of the building shortly after 6 a.m.
Calvery said authorities were unsure who fired the shots and with what kind of gun, though he said he believes they came from a rifle.
A portion of Interstate 395 -- which runs along the south side of the Pentagon -- was also shut down temporarily to conduct a search in the investigation. Maj. Chris Perrine, a public affairs officer for Defense Press Operations, said that other temporary road closures may be necessary as the probe continues.
Several law enforcement agencies, including the FBI and Pentagon police, are looking at surveillance footage and doing ballistics tests of the bullets found thus far, among other measures.
Tuesday's shooting follows a similar incident overnight Sunday at the National Museum of the Marine Corps in Triangle, Virginia, near the entrance of Marine Corps Base Quantico.
Lin Ezell, the museum's director, told CNN Tuesday that investigators have determined that one or more assailants used rifles to fire 10 bullets at the building -- five hitting glass windows, the rest hitting metal panels. Authorities haven't pinned down exactly where the shots were fired from, though they believe they came from Interstate 95 or nearby.
No one was hurt in that incident, which occurred between 12:15 a.m. and 5 a.m. when the building was unoccupied, Ezell said. No one has claimed responsibility and there were no known threats prior to the shooting, she added. Military police and the Prince William County Police Department are investigating that incident.
Asked if there were any ties between that and the Pentagon shootings, a law enforcement official said that "except for the similarity in the incidents -- windows shot out at military facilities -- there is nothing to connect these incidents at this point."
Both shots appeared to have been fired from high-velocity rifles, the official said. But the FBI, which is helping with ballistics tests related to the Pentagon shooting, has yet to see evidence from the Marine Museum shooting.
Tuesday's shooting was the first such incident at the U.S. Defense Department headquarters since March, when John Patrick Bedell pulled a gun from his pocket and began shooting. Bedell, who had a history of mental health problems, was later shot and killed, while two Pentagon police officers received superficial injuries in the incident.
CNN's Carol Cratty contributed to this report.