Nearly three years after the Sept. 11 attacks, two key elements of the Bush administration's effort to bolster airport security (search) remain works in progress: more rigorous background checks of passengers and a better way to check for explosives in luggage.

A plan to prescreen air travelers for terrorist connections, once described by the administration as an urgent need, has been sent back to the drawing board. And only eight of 441 commercial airports have systems recognized as the best at quickly and effectively screening checked baggage.

The reasons for the delays are varied. Technology problems and privacy concerns doomed the passenger prescreening program, while the enormous cost — an estimated $5 billion — has held up progress installing large bomb-screening machines in airports.

Rep. John Mica (search), chairman of the House aviation subcommittee, worries that the political pressure needed for such initiatives is waning.

"The further away you get from 9/11, the louder the voices become for a normal approach to security," said Mica, R-Fla.

The Transportation Security Administration said in January that the prescreening project — called Computer-Assisted Passenger Pre-screening System, or CAPPS II (search) — could be up and running this summer. But the agency never was able to allay concerns about privacy, and last week acting TSA Administrator David Stone said CAPPS II would be "reshaped and repackaged."

Airlines have been responsible for determining which passengers get extra attention at security checkpoints. Since the 1980s, they've been using such criteria as whether a passenger is flying one-way or pays for a ticket with cash. Screeners also select passengers for extra attention.

That system, though, is viewed as ineffective because it flags too many people and doesn't confirm their identities.

CAPPS II would have used commercial databases to verify passengers' identity. Privacy advocates and airlines were concerned about the invasiveness of such data-mining, which could wrongly suggest people are terrorists because of inaccurate data.

"There needs to be due process rights," said David Sobel, general counsel for the Electronic Privacy Information Center.

Technical problems hurt CAPPS II, according to Mica.

"You get this incredible amount of information, and sorting through it is very, very difficult."

An aide to Homeland Security Secretary Tom Ridge said the newest version of passenger prescreening would still check names against watch lists, but it's unclear what data would be used.

"No final decisions have been made," said Homeland Security spokesman Dennis Murphy.

Murphy said that CAPPS II was just part of a comprehensive approach to aviation security.

The number of federal air marshals grew to thousands from just a few dozen on Sept. 11, 2001. Airlines installed bulletproof cockpit doors and some pilots now fly while carrying weapons. The private screening work force, characterized by low morale and high turnover, was replaced with federal screeners who are better trained and better paid.

A law passed after the terrorist attacks requires all checked luggage to be screened using electronic devices.

Installing the best systems for doing so is proving a challenge.

According to reports by the Homeland Security Department's inspector general and congressional auditors, bomb-screening machines do a much better job detecting weapons in checked baggage if they're integrated with airports' in-line baggage-handling systems.

The standalone machines require screeners to load luggage by hand, which is a distraction. In contrast, bags that pass through the fully integrated systems can be easily reviewed multiple times by a remote operator.

But only eight airports have done that: in Boston; Boise, Idaho; Manchester, N.H.; Jacksonville, Fla.; Lexington, Ky.; Orange County, Calif.; Tulsa, Okla., and Tampa, Fla.

The rest use more labor-intensive, more error-prone approaches. Screeners either use handheld devices or hand-feed luggage into big bomb-screening machines sitting in terminal lobbies.

In May, the Transportation Security Administration wasn't electronically screening all checked bags for bombs at airports, as Congress ordered. At some airports, the agency hasn't been able to screen 100 percent of checked baggage 100 percent of the time for hundreds of consecutive days, according to congressional investigators.

TSA Acting Assistant Administrator Randy Null told the congressional subcommittee that half weren't complying with the law because of equipment problems and half because of staffing shortages or lack of trained screeners.

Oregon Rep. Peter DeFazio, ranking Democrat on the aviation subcommittee, said investing in more in-line machines would help solve the problem of inadequate staffing.

The total cost is estimated at about $5 billion, but the administration is requesting only $250 million this year.