federal agents

Federal agents are zeroing in on how the Boston Marathon bombing was carried out, but there's still no word on who did it and why.

An intelligence bulletin issued to law enforcement and released late Tuesday includes a picture of a mangled pressure cooker and a torn black bag. The FBI says evidence indicates the bomber packed a pressure cooker with explosives, nails and ball bearings and hid it in a back pack. The agency says the second bomb was in a metal container, but there isn't enough evidence to indicate that it was a pressure cooker.