Washington (CNN) -- More bullets struck the Pentagon in a shooting Tuesday than initially thought, officials said.
"It has been determined that at least six shots were fired," Steven Calvery, director of the Pentagon Force Protection Agency, said in a statement Wednesday. "As previously stated, two exterior windows were impacted by bullets. Upon investigation, it has been determined that four other bullets hit the Pentagon's facade."
Calvery said the investigation was ongoing and the FBI was evaluating evidence, including video footage from the Pentagon and surrounding buildings and roads as well as ballistics.
On Tuesday, Calvery told reporters he thought the shooting, which took place early Tuesday at the Defense Department's headquarters, was "a random incident."
"We are looking at all the possibilities," he said. "What we have is an isolated incident so far."
Pentagon police officers as well as construction workers in the area heard at least five shots fired around 4:50 a.m. Tuesday, Pentagon officials said. By midday, authorities had found two bullet fragments in third- and fourth-floor windows on the south side of the building, Calvery said. That part of the building is in the process of being renovated and was empty at the time of the shooting.
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.
Tuesday's shooting followed 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, said 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 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 and 5 a.m. when the building was unoccupied, Ezell said. No one has claimed responsibility, and there were no known threats before the shooting, she added. Military police and the Prince William County police are investigating that incident.
Tuesday's shooting was the first such incident at the 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.
CNN's Larry Shaughnessy and Carol Cratty contributed to this report.