kayaking down the streets of chicago. floods, cars swallowed in a big hole. the earth no match for the deluge. and the heart of a hero. from boston to texas, the ordinary americans who rushed toward danger. is there a super hero in every one of us? good evening to you, and tonight from texas to boston, we are a nation in pursuit of answers. and as we come on the air tonight, there is stunning news out of boston. the fbi releasing surveillance tapes. two people called persons of interest in the marathon-day bombings. the fbi considers them armed, dangerous, asked that no one approach them. but they ask all members of the public to look at these tapes, to study them in detail and help. abc s chief investigative correspondent, brian ross, will take us frame by frame through the clues. brian? reporter: good evening. federal authorities now believe that a team of at least two men was responsible for the bombings here. as you say, described as armed and extremely dangerous. t