But there are far superior machines available right now that could give screeners 3-D views of crammed carry-ons, spot weapons concealed under clothing, and differentiate dangerous liquids from the bottles of designer water that pile up at the portal. Among them:

Whole body imagers: These could detect prohibited items without intrusive physical pat-downs. As of late last year, they were being field-tested at only one airport—Phoenix. The cost: $100,000 to $150,000 apiece.

Bottled liquids scanners: Some 200 are now being used, but they are still not widely available. The cost: $20,000 to $25,000 apiece.

Advanced X-ray machines and automated explosives detection scanners: These use 3-D imagery to better spot bomb parts; few airports are equipped with them. The cost: $50,000 to $150,000 apiece.

The United Kingdom is well ahead of the United States in employing the newer machines. It strictly limited hand luggage to one piece per passenger in 2006, right after local authorities uncovered a plot to take down several aircraft using liquid explosives. But thanks to new state-of-the-art X-ray machines just installed at most major airports (including Heathrow but not Gatwick), the country has now eased the rules to allow two carry-ons. Damon Hunt, a spokesman for the British Airports Authority, which runs the airports and provides security services, notes that the multiple views afforded by the dual-screen technology eliminate the need to rescreen overstuffed bags and that the equipment makes the checkpoint process more secure and faster.

Absent better machinery, the only sure way to improve screeners' ability to detect contraband would be to increase the number of random physical searches and to make them far more intrusive than the gropings passengers now complain about, says GAO lead investigator Gregory Kutz. Increased pat-downs would also require more TSA manpower, something already in short supply. Congress has not altered a four-year-old cap on the total number of screeners, which remains at 43,000 full-time positions, despite a 19 percent surge in the number of passengers. In recent months, TSA staff were stretched even thinner when the agency created new positions—including "behavior detection officers," who are trained to spot signs of nervousness or guilt in passengers. Some 600 such screeners are now in place at major airports. Congress granted the TSA some additional money for these slots, but they were filled primarily by experienced screeners, creating a brain drain at certain checkpoints. The TSA says that it is dealing with the staffing constraints by hiring more part-time workers to fill in during peak hours; part-timers, however, have a higher-than-average attrition rate. According to some TSA employees, a more serious issue is that supervisors discourage screeners from speaking out when they spot vulnerabilities. The TSA has since said that it plans to increase whistle-blower protections for employees.