The fortress has not been well-fortified for some time.
Planes fly over it. Buses drive up to it. The subway stops under it. And until this year, commercial trucks were able to drive right into it.
The Pentagon, built to be as strong and impenetrable as this country always hoped its military would be, has been the focus of intense concern for more than a decade among the people charged with keeping its workers and its secrets secure.
Defense officials Saturday credited safety measures instituted as part of a massive renovation effort, which began in 1994, with saving lives and preventing further damage from Tuesday's crash of a hijacked American Airlines jet.
In addition to the 64 people killed aboard the plane, 125 civilians and military personnel died from the impact and resulting inferno.
But the renovation--estimated to end up costing more than $1 billion--is still less than one-fifth complete. On top of that, it will cost at least $300 million to repair the damage from the attack, officials estimate. If plans for the remodeling go forward on schedule, it won't be done until 2014.
And even when it is, the vast complex that houses the military's central nervous system will remain vulnerable to attack in critical and uncorrectable ways, defense officials said.
"The existing Pentagon is not compliant with fire safety codes. The existing Pentagon is not compliant with any codes," said Lee Evey, program manager for the renovation.
"It is one of the most likely terrorist targets in town. It is an old building. More than 25,000 people work here. There's no way we could design this building or any building to be 100% protected."
The weight of the challenge began to emerge Saturday, even as fire and rescue workers and construction crews continued to move gingerly through the collapsed, torn, burnt and flooded areas of the building looking for remains and for clues that could help in the investigation.
Video footage shot by Air Force combat photographers showed the building missing an entire section, like a giant cake with a slice cut out. Melted plastic hung like icicles from fluorescent light fixtures. Fire crews moved through the muck wearing protective suits, respirators, goggles and helmets. A pair of reading glasses sat on a desk in a room that had otherwise been blown away. Next to them was a bowl, its brightly colored candies spilling out.
But the video also showed how much worse the damage could have been.
American Airlines Flight 77 struck a portion of the building that had already been renovated. It was the only area of the Pentagon with a sprinkler system, and it had been reconstructed with a web of steel columns and bars to withstand bomb blasts. The steel reinforcement, bolted together to form a continuous structure through all of the Pentagon's five floors, kept that section of the building from collapsing for 30 minutes--enough time for hundreds of people to crawl out to safety.
The area struck by the plane also had blast-resistant windows--2 inches thick and 2,500 pounds each--that stayed intact during the crash and fire. It had fire doors that opened automatically and newly built exits that allowed people to get out.
"This was a terrible tragedy, but I'm here to tell you that if we had not undertaken these efforts in the building, this could have been much, much worse," Evey said. "The fact that they happened to hit an area that we had built so sturdily was a wonderful gift."
The rest of the Pentagon would not have fared as well.
The fire that swept through the building caused the greatest damage in an unrenovated section with no sprinkler system, heavy windows or steel reinforcements. But many of the offices there were empty in anticipation of the renovation.
While perhaps 4,500 people normally would have been working in the hardest-hit areas, because of the renovation work only about 800 were there Tuesday, officials said.
The building is packed with thousands of tons of asbestos, brushed with lead-based paint and constructed with mercury and PCBs. When ground was broken on the building--eerily, on Sept. 11, 1941, exactly 60 years before Tuesday's attack--it was a state-of-the-art bunker. Now it is a still-vulnerable target, both to an attack and to security breaches that could give enemies access to U.S. military secrets.
"I thought in general our security needed to be improved; I worked very hard at it," said John Hamre, who as a deputy secretary of defense in the Clinton administration made security a focus of his tenure.
"I always thought that was the third-most likely terrorist target in the town, behind the White House and the Capitol," Hamre said. "It remains a potent target because of the larger symbol it conveys that the American military is susceptible to attack at its heart, at its central nervous system."
The Pentagon has always sought to toe an uneasy line between security and public access. It sits between a network of freeways and just a few miles from one of the area's major airports.
Planners built a subway stop next to the Pentagon with an escalator that leads straight up to the building to allow employees to get to work. Today, more than 15,000 ride the Metro to the building every day.
But a bomb planted on the Metro platform would send much of its blast into the Pentagon. The subway stop is also vulnerable to a biological weapon or a gas attack.
"The subway was a major concern and it remains a major concern," Hamre said. "Right now, if there was an explosion, the blast effects would just be pulled up into the building like a chimney."