(CNN) -- A man coolly and calmly approached the screening area outside the Pentagon Thursday evening and opened fire, grazing two Pentagon police officers before they returned fire, critically wounding him, officials said.

The incident happened at 6:40 p.m., when the man wearing a coat -- with "no real emotion in his face" -- approached the officers outside the Pentagon Metro station, said Pentagon Police Chief Richard S. Keevill.

"As the officers started to ask him for his pass to get into the Pentagon, he drew a weapon from his pocket and started shooting immediately at the officers" from a few feet away, Keevill told reporters.

"He drew a gun and just started shooting immediately."

The two Pentagon Force Protection Agency officers returned fire with their semi-automatic Glock .40-caliber weapons and the suspect, thought to be a U.S. citizen, was critically wounded, Keevill said. He praised the police officers for acting "quickly and decisively to neutralize him as a threat" without hurting anyone else.

Asked how many shots were fired, he said, "Many."

Keevill would not identify the man.

The Pentagon Force Protection Agency is the Pentagon's police department.

Pentagon entrances were locked briefly but all were reopened with the exception of the Pentagon Metro entrance, the Pentagon said in a statement.

Lisa McDonald, a spokeswoman for George Washington Hospital, said three people were being treated there -- both officers and the suspect.

The Pentagon police department, the Arlington County Police Department, U.S. Secret Service and the FBI were all involved in the investigation, Keevill said.

The Pentagon is one of the largest office buildings in the world with three times the floor space of New York's Empire State Building, according to its official Web site.

Some 23,000 military and civilian employees work there.

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Though it contains 17.5 miles of corridors, a person can walk between any two points in the World War II-era building in no more than seven minutes.

CNN's Mike Ahlers, Larry Shaughnessy and Jeanne Meserve contributed to this story.