Five years after Congress set a deadline for requiring all U.S.- bound shipping containers to be X-rayed overseas for nuclear weapons, customs officials have all but given up on the goal.

Customs and Border Protection officials scanned with X-ray or gamma-ray machines 473,380, or 4.1 percent, of the 11.5 million containers shipped in the fiscal year ended Sept. 30, according to the agency. That's essentially the same percentage of containers that were scanned in 2007, the year a Democratic-controlled Congress mandated that agents start vetting every container.

Screening 100 percent of incoming containers would be nearly impossible to implement now, cause huge delays and be less cost- effective than focusing only on suspicious cargo, customs officials say, even as the law's supporters insist the mandate is the only way to ensure the safety of the shipping system.

"It's not necessarily a good use of resources to spend time and effort on ships that pose no risk," said Jayson Ahern, the agency's acting commissioner until January 2010.

Earlier this year, Homeland Security Secretary Janet Napolitano, who oversees the customs agency, granted a two-year waiver from the requirement. Defending the decision last month before the House Homeland Security Committee, Ms. Napolitano said the mandate isn't "practicable" or "affordable" now.

The secretary's comments signal a willingness to challenge the wisdom of the screening requirement, said Jessica Zuckerman, a research associate at the Heritage Foundation, a Washington-based policy group.

The agency does "not see 100 percent as a feasible or a common- sense strategy," she said.

Lawmakers favoring the mandate say they're concerned about terrorists detonating a nuclear or dirty bomb at a port, killing workers and rendering the facility and surrounding area uninhabitable for years.

The selective approach "will not prevent all potential attacks inside the U.S., as it is not comprehensive and is subject to human error and weaknesses in our defense systems," said Rep. Jerrold Nadler, D-N.Y., who co-sponsored the scanning mandate.

Customs officials and business lobbying groups such as the U.S. Chamber of Commerce have said X-raying all containers would be too expensive, require cooperation from foreign countries that isn't forthcoming and delay the flow of shipments.

Ms. Napolitano at the House hearing cited a lack of agreements with some foreign countries to X-ray cargoes overseas.

"There are a lot of foreign ports it's just physically not available to us to do that," she said.

As of June 30, customs had scanned 342,527, or 3.8 percent, of the 9 million containers shipped so far this year.

Officials are now getting better information to help them zero in on containers that might have contraband, said Joanne Ferreira, a Customs and Border Protection spokeswoman.

"Targeting has become more refined," Ms. Ferreira said.

Information about the targeting's success isn't public for security reasons, said Ian Phillips, a customs spokesman.

Still, the Homeland Security Department's inspector general, its chief internal investigator, found problems with the system, including a lack of uniform procedures and data needed to decide if a shipment should be cleared or held.

So much commerce flows through ports that any interruption stemming from an attack would reverberate throughout the worldwide supply system, said Stephen Flynn, founding co-director of the George J. Kostas Research Institute for Homeland Security at Northeastern University in Boston. …