Science correspondent Miles O'Brien examines the threat that radioactive "dirty bombs"could pose to cities in the U.S., and what's being done to prevent a radiological attack from happening.

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In a sting operation orchestrated by the federal government, congressional investigators who set up a bogus company were able to purchase materials for a dirty bomb, despite Nuclear Regulatory Commission oversight.

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Two teams of government investigators testing the effectiveness of radiation monitors at U.S. borders were able to smuggle in enough radioactive material to make two "dirty bombs," a federal report released Monday said.

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