WASHINGTON -- An FBI investigation is under way to determine if the Boston Marathon bombing suspects acted alone.
So far, U.S. officials say there's no evidence of a wider plot.
But Rep. Michael McCaul, who chairs the House Homeland Security Committee, tells Fox News the type of explosive devices used leads him to believe someone else trained the bombers.
Investigators looking at bombing suspects' mom
The mother of the Boston Marathon bombing suspects is drawing increased attention after U.S. officials said Russian authorities intercepted her phone calls, including one in which she vaguely discussed jihad with one of her sons.
Russian-born Zubeidat Tsarnaeva says she's no terrorist, just someone who found a deeper spirituality.