high school student sidney corkran is to his left. >> it was a great vibe. everyone was just being nice and cheering them on like, you can do it, keep going. >> dzokhar stops behind several families with small children, including the richards, with 11-year-old henry, 6 jeerld jane and 12-year-old martin. 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 lieu. >> she walked into frame just before the bomb goes off. you see he turn to look toward the finish line where the first bomb went off and then you see her continuing to walk across the frame and then the second bomb goes off. >> the blast levels spectators and separates families. >> my mind always goes to the richard family. bill richard who saw the first bomb go off and thought i need to get m