A gunman coolly drew a weapon from his pocket and opened fire at the teeming subway entrance to the Pentagon complex Thursday evening, wounding two police officers before being shot and critically wounded, officials said.

The two officers suffered grazing wounds that were not life-threatening and were being treated in hospital, said Richard Keevill, chief of Pentagon police.

The suspect, believed to be a U.S. citizen, walked up to a security checkpoint at the Pentagon in an apparent attempt to get inside the massively fortified Defense Department headquarters, at about 6:40 p.m. "He just reached in his pocket, pulled out a gun and started shooting," Keevill said. "He walked up very cool. He had no real emotion on his face." The Pentagon officers returned fire with semiautomatic weapons.

Of the suspect, the chief said, "His injury is pretty critical."

The assault at the very threshold of the Pentagon _ the U.S. capital's ground zero on Sept. 11, 2001 _ came four months after a deadly attack on the Army's Fort Hood, Texas, base allegedly by a U.S. Army psychiatrist with radical Islamic leanings. In the immediate aftermath Thursday, investigators did not think terrorism was involved but were not ruling that out and did not discuss possible motives.

Law enforcement sources identified the suspect as John Patrick Bedell, 36. They also said they were investigating whether a second person was somehow involved. No further information was available.

The subway station is immediately adjacent to the Pentagon building, a five-sided northern Virginia colossus across the Potomac River from Washington. Since a redesign following the Sept. 11, 2001, terrorist attack on the Pentagon, riders can no longer disembark directly into the building. Riders take a long escalator ride to the surface from the underground station, then pass through a security check outside the doors of the building, where further security awaits.

After the attack, all Pentagon entrances were secured, then all were reopened except one from the subway, said Pentagon spokesman Bryan Whitman.

Keevill said the gunman gave no clue to the officers at the checkpoint about what he was going to do.

"There was no distress," he said. "When he reached into his pocket they assumed he was going to get a pass and he came up with a gun."

"He wasn't pretending to be anyone. He was wearing a coat and walked up and just started shooting."

A Pentagon official working late in the building said people inside first heard of the shooting on television. They were later told the building was locked down and to stay in place.

Then at around 7:30 p.m. local time, they heard an announcement on the public address system that they could leave through Corridor 3 _ one widely used to get access to one of the parking lots.

"We really don't know anything, just that we can leave now through that corridor," one official said on condition of anonymity because he wasn't authorized to speak about the incident.

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Associated Press writers, Pauline Jelinek, Devlin Barrett, Eileen Sullivan, Matt Apuzzo, Anne Gearan and Nafeesa Syeed contributed to this report.