people. we were collecting pieces of shrapnel inside the bombs. reporter: day three, a break in the case. of the surveillance videos from businesses and spectators, a man in a white ball cap at the second blast site. he slides that backpack off his shoulder and stands and mills around. a short time later, he makes a cell phone call. very shortly thereafter, you hear the first bomb go off. reporter: day four, the fbi asks the public for help finding two men, later identified as tamerlan and dzhokhar tsarnaev, now on the run. officials say the brothers execute m.i.t. police officer sean collier, carjack an suv and get into a shootout with police. watertown police chief ed devo says tamerlan is shot but manages to reload about four times.
around. a short time later, maybe 15 minutes later he makes a cell phone call. very shortly thereafter you hear the first bomb go off. reporter: day four the fbi asks the public for help later identified as tamerlan and dzhokhar tsarnaev. now on the run. the brothers execute m.i.t. police officer shawn collier, car jack an suv and get into a shootout with police. watertown police chief ed devo says 26-year-old tamerlan tsarnaev is shot but manages to reload four times. he runs out of ammunition and throws his gun at my sergeant and he starts to run. reporter: officers tackle tamerlan and tre try to get his brother. he s lodged under the stolen suv and he smashes into our cruisers. reporter: by sunrise friday millions in the boston area are in lockdown. when it is lifted that night a