Dirty Bomb?

When I first saw the story of the dirty bomb in Boston plot and the fact that it was all based on an anonymous tip I pretty much ignored the story. So this US News Article came as a bit of a surprise:

Police carrying radiation detectors patrolled Boston's subway system on Friday after the FBI added another 10 names to a list of people it wants to question over a reported 'dirty bomb' plot in the city. Authorities reassured area residents that there was no cause for panic two days after an uncorroborated tip triggered a Federal Bureau of Investigation manhunt. Media reports spoke of threats to explode a so-called 'dirty bomb' which disperses low-level radioactive material. "

Law enforcement officials seem to be taking this threat very seriously, perhaps simply out of an abundance of caution. Still, the effort here seems greater than I would expect from the information we know.