former assistant secretary of homeland security. the other piece is building a strong case and making sure that case doesn t fall apart. that s why the indictment released this week was very, very careful to say look, we know he used weapons of mass destruction, we know he killed people, and that s enough to give him the death penalty. reporter: to build the case and find the answers, hundreds of federal investigators are now working around the clock, digging through debris, analyzing bits of bombs in one of the nation s largest terrorist investigations since 9/11. neighborhoods like this one across boston are being visited by the fbi. this man says five fbi agents showed up at his door to question his son about tweets sent to the younger brother. and on the college campus where dzhokhar went to school, friends
accelerating up up laurel street towards the police, towards the vehicles, basically flooring it. somebody at the last minute yelled get out of the way. and they dove out of the way as he came roaring through and ran over his brother. reporter: as tamerlan lies dying in the street, 19-year-old dzhokhar tsarnaev drives straight through a police barricade and escapes. the car that he abandoned a little further down the street there, was blood in that vehicle. so we knew he was wounded. we just didn t know how bad. reporter: wounded and on the loose. at dawn on friday an entire region is put on lockdown, as a convoy of law enforcement personnel and equipment rolls into watertown. we re asking people to shelter in place, in other words, to stay indoors with their doors locked. reporter: during the day friday door to door, room to room searches turn up empty, and
school, college, from boxing now realize they never really knew them at all. the two brothers, outwardly so different, on april 15th are accused of acting together to bomb the boston marathon. incredibly, just days after the bombings, dzhokhar was spotted back on campus. betancourt saw him at the gym. we talked about the bombing for like about five minutes. he seemed very nonchalant. he didn t seem like nervous or anything. he seemed a little maybe tired and off. i asked him how he s doing. he was like yeah, i haven t been doing much lately, so i decided to come to the gym. reporter: betancourt says dzhokhar s reaction wasn t surprising. at the time. i was just talking about how, yeah, in iraq and afghanistan these th hhe it s crazy w is would happen. and he just said he was like, yeah, tragedies happen, man. these things happen around the world. like it s crazy. reporter: for julian pollard he hadn t thought much of his former boxing buddy until last week.
after the second world war tens of thousands of chechens perished in a mass deportation ordered by joseph stalin. in the 1990s chechens fought and won a bloody war for independence from russia. it was during this violence that the younger tsarnaev brother dzhokhar was born and given the name of a chechen leader. it s a sure sign of chechen patriotism in the family that in 1993 they called their little son dzhokhar, after dzhokhar dudaev, who was the pro-independence president. reporter: but chechen independence wouldn t last. another war with russia would kill thousands. and chechen extremists would wage a campaign of terror across russia, killing 186 children at a school in beslan, and more than 100 theatergoers in moscow. the tsarnaev family escaped the violence of the region by making
strong case and making sure that case doesn t fall apart. that s why the indictment released this week was very, very careful to say look, we know he used weapons of mass destruction, we know he killed people, and that s enough to give him the death penalty. reporter: to build the case and find the answers, hundreds of federal investigators are now working around the clock, digging through debris, analyzing bits of bombs in one of the nation s largest terrorist investigations since 9/11. neighborhoods like this one across boston are being visited by the fbi. this man says five fbi agents showed up at his door to question his son about tweets sent to the younger brother. and on the college campus where dzhokhar went to school, friends there tell cnn they too have been visited by the fbi. up-front next, breaking