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Could terrorists smuggle dirty bombs into U.S.?

In an exclusive report, congressional sources tell NBC News that undercover investigators were able to bring enough material for two dirty bombs into the country. NBC's Lisa Myers has the story.

The port of Long Beach, Calif. — among the busiest in the nation — is a key line of homeland defense. Some 4.5 million shipping containers pass through each year.

Big radiation portal monitors scan some — but not all — containers for traces of nuclear or radiological material as they leave the port. But, four-and-a-half years after 9/11, Senate investigators say only 39 percent of all containers entering the U.S. are screened for nuclear material. Many ports, including the third-largest, Miami, still have only handheld detection devices of little value.

“We still have massive blind spots in our ability to prevent nuclear material from being smuggled into this country,” says Sen. Norm Coleman, R-Minn.

Coleman says the Department of Homeland Security still is not moving nearly fast enough.

A report by the Government Accountability Office concludes DHS is two years behind schedule in installing radiation monitors in ports and not likely to have them all done, even by 2009.

DHS has made more progress installing detection equipment at the borders, but there investigators found another hole in the system. Coleman tells NBC News that undercover GAO investigators were able to bring enough radioactive material into the U.S. to make two dirty bombs — penetrating both the northern and southern borders. Monitors detected the radiological material, but undercover agents produced fake papers and got the material in.

“They were able to use counterfeit documents they got off a basic program on a computer,” Coleman says.

DHS officials say they are now looking at how to plug that hole. But they insist significant progress has been made toward securing ports.

“We feel that at seaports that we're going to have a very suitable defense by the end of 2007,” says Vayl Oxford, the director of the Domestic Nuclear Detection Office, which operates under DHS.

Stephen Flynn, a top port security expert, says that's an illusion, that the new radiation monitors aren't enough.

“They're not enough because they still can’t help us find a nuclear weapon,” Flynn says, “and they can’t help us find highly enriched uranium.”

Experts say that without added technology and much greater urgency, Americans will remain vulnerable to this very real threat.

Lisa Myers is NBC’s senior investigative correspondent.