they want to shoot me. reporter: before the marathon attack three days earlier. meng tefred that tamerlan tsarnaev pointed a gun at him and said do you know the boston marathon explosion? i did it and i just killed a policeman in cambridge. all units down. all reporter: shot once between the eyes. surveillance video shows the brake lights flash as tries to steal collier s gun. the confrontation lasts 50 seconds, before the brothers run away. it sounds like someone is hitting a trash can really loud. reporter: later, the prosecutors in pain staking detail showing how the investigation unfolded with the fbi releasing these images of the suspected bombers. two men in baseball hats carrying backpacks walking