"They are really not weapons of mass destruction, they are weapons of mass disruption," Gregory Kutz, managing director of special investigations for the Government Accountability Office told me. "They wouldn't necessarily have enough radiation to kill anyone, but they could require the shutdown of potentially large parts of the city."

Kutz was describing the sort of device terrorists could construct if they got their hands on the same type and volume of radioactive material that two sets of GAO agents working at Kutz's direction smuggled across the U.S.-Mexico and U.S.-Canada borders in a covert test conducted on Dec. 14.

The two groups, Kutz told a Senate subcommittee in written testimony this week, each smuggled what the National Institutes of Standards and Technology determined was enough radioactive material to construct one "dirty bomb" apiece.

To demonstrate how easily terrorists could purchase the material to make these bombs, Kutz's investigators created a fictitious company based in Washington, D.C. The company ordered a portion -- but not all -- of the radioactive material needed for a bomb from a U.S. supplier over the telephone. They told the supplier they wanted the material to test personal-radiation-detection pagers (like those used by the U.S. Border Patrol). The supplier dropped the radioactive material in the mail.

"We did it just once to show that we could do it," Kutz told me. "We could have done it multiple times."

I asked Kutz: Where exactly in our capital city was the radioactive material mailed? "I can't tell you," he said. "I can just tell you that it was an address in Washington, D.C."

Kutz's agents then coordinated with the appropriate authorities to conduct their border-entry tests from Canada and Mexico. Using ordinary rental cars -- with the radioactive material stowed in the trunks -- they simultaneously approached a port of entry on each border.

"We came through the portal monitors at one location in the north and one location in the south, at the same time, and with the same name of a company, and the same amount of material," Kutz told me. "Our objectives were to determine whether the radiation portal monitors worked, first of all, so we had enough materials to actually set them off. Our second objective was to observe the reaction of CBP (Customs and Border Patrol) inspectors to our test. Then, our third objective was to see if we could beat the system with a ruse."

The monitors worked, and the CBP personnel did their jobs by the book. But then they fell for the ruse: The GAO agents produced counterfeit NRC documents indicating they were authorized to bring the material across the border. The customs agents did not have the means to check the authenticity of the documents.

Would tightening procedures for selling radioactive materials via mail or for authenticating NRC documents at our borders necessarily stop terrorists from getting the materials for a dirty bomb into the United States? No. Our border remains wide open in other ways.

I asked Kutz why someone couldn't take a four-wheel-drive vehicle loaded with radioactive material across the desert where there are no monitors. "We never tried that before, but it's possible" he said. Continued...