The attack unfolded at the subway entrance to the massive Defense Department headquarters, as an eerie calm and silence were broken by the explosion of gunfire.
"He just reached in his pocket, pulled out a gun and started shooting" at point-blank range, said Richard Keevill, chief of Pentagon police. "He walked up very cool. He had no real emotion on his face."
The Pentagon officers returned fire with semiautomatic weapons, sending the shooter to the hospital with critical injuries. Beverly Fields, chief of staff of the D.C. medical examiner's office, confirmed the man's death and said his body arrived at her office shortly after midnight.
John Patrick Bedell, 36, of Hollister, Calif., was identified as the shooter. Officials said they'd found no immediate connection to terrorism but had not ruled it out.
Signs emerged that Bedell harbored ill feelings toward the government and the armed forces, and had questioned the circumstances behind the Sept. 11, 2001, terrorist attacks.
In an Internet posting, a user by the name JPatrickBedell wrote that he was "determined to see that justice is served" in the death of Marine Col. James Sabow, who was found dead in the backyard of his California home in 1991. The death was ruled a suicide but the case has long been the source of theories of a cover up.
The user named JPatrickBedell wrote the Sabow case was "a step toward establishing the truth of events such as the September 11 demolitions."
That same posting railed against the government's enforcement of marijuana laws and included links to the author's 2006 court case in Orange County, Calif., for cultivating marijuana and resisting a police officer. Court records available online show the date of birth on the case mentioned by the user JPatrickBedell matches that of the John Patrick Bedell suspected in the shooting.
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.
Hatred of the government motivated a man in Texas last month to fly a small plane into a building housing Internal Revenue Service offices, killing an IRS employee and himself.
Whatever the motive of Thursday's attack, the method resembled one in January in which a gunman walked up to the security entrance of a Las Vegas courthouse and opened fire with a shotgun, killing one officer and wounding another before being gunned down in a barrage of return fire.
President Barack Obama was getting FBI updates on the Pentagon shooting through his homeland security and counterterrorism adviser, John Brennan, White House spokesman Nick Shapiro said.
Law enforcement officials were also scrutinizing 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.
Transit officials said the station would remain closed at least part of the day Friday while the FBI continued its investigation.
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."
Ronald Domingues, 74, who lives next door to Bedell's parents in a gated golf course community in Hollister, said he doesn't know the family well. But he said Bedell sometimes lived with his parents and struck him "like a normal young man."
"He just seemed like a normal guy to me," Domingues said. "I wouldn't suspect he would be involved in anything like this."
Domingues described the neighborhood as middle-class. He said the Bedells live in a one story southwestern-style stucco home. The house was dark Thursday night.