>> dzokhar stops behind several families with small children, including the richards, with 12-year-old henry, 8-year-old martin and 7-year-old jane. and now dzokhar raises a phone to his ear. the plan is now in motion. >> what the hell was that? >> tamerlan detonates the first bomb. >> oh, my god, something blew up. >> in the ensuing chaos, dzokhar walks quickly away from his backpack. he leaves it on the ground, right near 8-year-old martin richard. behind the richards in a blue sweater is 23-year-old lingzi lu.