Authorities searched Tuesday for a gunman who fired shots at the Pentagon in the early morning, possibly using a high-powered rifle. No one was injured.

The shots were fired shortly before 5 a.m., shattering, but not penetrating, windows on the third and fourth floors, officials said. They said the walls of the building might have been struck.

"Right now, we are considering this a random event," said Pentagon Force Protection Agency Director Steven E. Calvery. "We can only process what we have."

Federal and local authorities spent the morning searching for evidence, closing adjacent Interstate 395 briefly and combing through grassy areas. The FBI also was examining bullet fragments lodged in the windows.

Officials said they think a high-velocity rifle was used but were not certain. Two windows were hit, but officials do not know how many shots were fired.

The Joint Terrorism Task Force is taking the lead in the investigation, authorities said.

The incident came a little more than seven months after a man opened fire at a Pentagon entrance. Officers Jeffery Amos, 46, and Marvin L. Carraway Jr., 44, suffered minor injuries in the March 4 shootout. The gunman, John P. Bedell, 36, who family members said suffered from mental problems, was killed.

Officials said officers in the south parking lot Tuesday heard the shots just before 5 a.m. and reported them to headquarters. The Pentagon was locked down, and entrances were closed as authorities swept the area. The entrances were reopened about 5:40 a.m.

"We did take prompt action in immediately closing down the reservation," Calvery said. He said no threats or intelligence pointed to the incident.

No one was inside the offices where windows were struck, officials said. They said those spaces are being renovated.

Authorities were gathering videotape from the Pentagon City area, searching rooftops of buildings, and interviewing nearby construction workers and others.

Officials said they did not know whether there was any connection to Monday's discovery of bullet holes in windows at the National Museum of the Marine Corps in Triangle, about 30 miles south of the Pentagon. They said investigators will examine both incidents.

About 23,000 Defense Department employees and contractors work at the Pentagon, which receives more than 1,000 visitors a day, according to the protection agency. With Metrorail and bus passengers, the agency said, 50,000 to 60,000 people on average pass through the Pentagon complex on workdays.

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