In 2001, a month after 19 hijackers slipped through security at three U.S. airports and turned four jetliners into guided missiles, Condé Nast Traveler reported on the government's ambitious six-part plan to make our skies secure. Now, half a decade and more than $20 billion in federal funds later, we team with top aviation experts to see how far we've come. What we uncover in our survey of eight of the nation's busiest airports shows that the job is far from done

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I am standing on the roof of a parking garage at an airport that serves a city considered to be one of the most likely terrorist targets in the United States. Airplanes pass overhead at regular intervals; the terminal of one of the largest airlines in the country is directly across the roadway. With me is Issy Boim, a former Israeli secret service agent and El Al sky marshal, who has advised dozens of major airlines and private jet companies about how to protect themselves against sabotage. He is no idle observer; Condé Nast Traveler has enlisted him to investigate the state of security at U.S. airports.

To assemble the big picture, Boim is assessing the vulnerability of eight major airports to all manner of potential threats, from explosives smuggled on board an unguarded aircraft to a missile attack on a plane during landing or takeoff. He is also paying particular attention to what he calls the "multiple layers" of security, a concentric configuration of safeguards that is designed to prevent an attack even if terrorists manage to breach one of them, as it appears he and I have just done: Although the rooftop parking area is closed, it was easy to gain access to it by foot. "Why isn't someone coming to ask me what I am doing here?" Boim wonders aloud.

"Could a terrorist take down a plane from here?" he says, watching a parade of aircraft alight on the nearby runway. "It would be difficult but not impossible."

Five years after 9/11, and following an upgrade of aviation security costing more than $20 billion, the federal government is still plagued by criticism from members of Congress, airline workers, and safety advocates who claim it hasn't achieved its goal of shoring up security at the 441 commercial airports in the United States. Some, including Congressman John Mica of Florida, argue that the effort has gone so far astray that we're not much safer than we were before. At a recent hearing of the House Aviation Subcommittee, which he chairs, Mica lambasted the Transportation Security Administration (TSA) for its "disgraceful" handling of its screening duties and warned that in the critical area of bomb detection, "the system is getting bogged down by its own inefficiencies, [and] there is growing evidence it does not even afford us more effective security screening." His damning assessment: "The vast majority of airports in the country are in a state of disarray today."

Many other U.S. aviation insiders agree. Although the government periodically sends teams of anonymous inspectors to test airport security, as it did before 9/11, the results are no longer made public, leaving Americans largely in the dark. Occasional leaks to the press, however, suggest that screeners continue to miss potential weapons, especially explosives; a report leaked last spring said that inspectors had been able to carry bomb-making materials past screeners at 21 airports.

Addressing weaknesses in the system is difficult, however, because apart from a federal "baseline" plan that each airport must follow, an airport's security is largely left to the discretion of state and local authorities. The result is a hodgepodge of measures that have produced vastly different security levels from one airport to the next. This absence of consistent standards, Boim says, is one of the biggest threats to security.