WASHINGTON - A gunman coolly drew a weapon from his pocket and opened fire at a security checkpoint into the Pentagon on Thursday in a point-blank attack that wounded two police officers before the suspect was critically shot.

The two officers suffered grazing wounds and were being treated in a hospital, said Richard Keevill, chief of Pentagon police. Authorities had no motive for the shooting.

The suspect, believed to be a U.S. citizen, walked up to the checkpoint at the Pentagon's subway entrance in an apparent attempt to get inside the massively fortified Defense Department headquarters. "He just reached in his pocket, pulled out a gun and started shooting" no more than five feet away, 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, post 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.

President Barack Obama was closely following the case with updates from the FBI through his homeland security and counterterrorism adviser, John Brennan," White House spokesman Nick Shapiro said.

Law enforcement officials identified the suspect as John Patrick Bedell, 36. They also said they were speaking with a second man, who might have accompanied the shooter, and were running his name through databases.

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 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. He said the subway entrance was likely to remain closed overnight at least.

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."

Keevill added: "We have layers of security and it worked. He never got inside the building to hurt anyone."

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., 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.

Associated Press writers Pauline Jelinek, Devlin Barrett, Matt Apuzzo, Anne Gearan, and Mike Gracia, Nafeesa Syeed and Philip Elliott contributed to this report.