>> that was the special agent incharge of the boston area. this image released from the fbi, joint bulletin issued to law enforcement. it shows the remains of a pressure cooker. the fbi says it's part of one of the detonated boks. it shows this image of an exploded backpack. the fbi is examining photos like this from nbc affiliate, whdh. it shows the marathon route before the second bomb detonated. here is a picture of the same location after the second bomb detonated. sources tell nbc news that the triggering mechanism appears to have had a battery pack and circuit board, elements of a sophisticated system. both elements were recovered at the scene.