Nine years after the Sept. 11 attacks and decades after hijackers first began to target passenger airliners, the United States has invested billions of dollars in an airport system that makes technology the last line of defense to intercept terrorists.

It has yet to catch one.

In every known recent attempt, terrorists have used a different tactic to evade the latest technology at airport checkpoints, only to be thwarted by information unearthed through intelligence work - or by alert passengers in flight.

The result is an emerging consensus among experts and lawmakers that the checkpoint-heavy approach - searching nearly every passenger - may not be the most effective.

Instead, many of them say, the system should focus more urgently on individuals, gathering a greater range of information about people to identify those most likely to present a real danger.

Scanners, pat-downs and bomb-sniffing dogs are all vital parts of the process but should be integrated into a multilayered system that includes far-reaching, computer-filtered data about people, along with face-to-face monitoring by the modern equivalent of a beat cop, several officials and experts said. Technology matters, they said, but it is akin to putting up a series of picket fences for terrorists to evade.

U.S. officials and lawmakers acknowledge that broader revisions may be necessary, saying it is only a matter of time before the airport security apparatus fails.

"Let's be honest: We've been lucky the last few times," said Senate Homeland Security Committee Chairman Joseph I. Lieberman (I-Conn.). "With the Christmas Day bomber over Detroit and the Times Square bomber and the air cargo attempt, they did not succeed, but that's because of their own inadequacies, not because we were able to stop them."

As a result of those attempts, passengers must surrender sharp objects (a response to the Sept. 11 attacks) and slip off their shoes (a response to the 2001 would-be shoe bomber). They must remove liquids from their bags (a result of a 2006 plot to blow up planes), and, as of a few weeks ago, they must submit to body scans or pat-downs (a process accelerated by the attempted airline bombing last Christmas Day).

Yet lawmakers and government reports question the capability of some specific measures. Year after year, undercover testers manage to sneak loaded weapons past screeners in embarrassing evasions. More broadly, skeptics describe the extreme focus on airport checkpoints as incomplete, too often focused on the last attack rather than the next one.

Even Transportation Security Administration head John S. Pistole, in an interview, described his agency as merely a "last line of defense on a continuum of government national security efforts."

Like others interviewed, Pistole said he hopes to move to a more intelligence-based system, but said the previous attacks could never be ignored.