Federal authorities are combing the woods near the UMass-Dartmouth dorm room of Boston bombing suspect Dzhokhar Tsarnaev, Fox News has learned, as well as the areas around the buildings where he attended classes.

Megyn Kelly reported on America Live that the Bureau of Alcohol, Tobacco and Firearms is taking the lead on the search, but authorities can't say what they're looking for.

Chief intelligence correspondent Catherine Herridge "hypothesized" on the program, saying, "When you build a bomb, you have to have a way to test them to see if they're stable or unstable," she said, adding that such a test is important because each device is a little bit different. "This may well be a search in connection to it."

A national security source confirmed to Fox News that the brothers Tsarnaev had made four different kinds of IEDs or explosives, and all were effective.