Is This the Hat Worn by Boston Bombing Suspect #1?
If so, it's a Bridgestone golf cap.
One of the suspects' hats may be this Bridgestone Golf hat:
Here's a second side-by-side of the back of the Bridgestone cap and a photo that Reddit user streetsim says he or she took at the marathon (text is also visible on the back of Suspect 1's hat in the video the FBI released):
See the rest of the photos the FBI released of the Boston Marathon bombing suspects here.