Dateline | April 24, 2013
>>> wanted to find out what could have possessed these two men from russia that had called america a home for more than a decade.
>> they became convinced that the wars in afghanistan and iraq were wars against islam and they wanted to do something in return, was the main motivation.
>> dzhokhar, from his hospital bed, told investigators that he did it.
>> in essence, he confessed.
>> they believe they know the who. the question is still the why and to some extent the how.
>> and one big why.
>> what led the radicalization of the brothers and that led to the question.
>> perhaps some clues come from tamerlan 's friends in the boston area .
>> when i met him in 2009 , he was --
>> he was a gifted boxer, and he and julian pollard became friends.
>> flashy guy, confident guy, and the guy was humbled and talked to me about his faith.
>> the fbi was told in 2011 by the russian government that tamerlan was a follower of radical islam and a strong believer who was planning to travel to russia and join unspecified underground groups, the fbi opened up a case file on tamerlan .
>> they looked into u.s. internal terrorism databases and found nothing. and they looked at whether he was in touch with suspected terrorists that were in the databases, and found nothing. and then they decided to interview him and members of his family in cambridge. and came away thinking we got nothing on this guy. so the fbi says they went back to the russians and said we're coming up empty, what more do you have? and they said the russians never responded. so in essence, the case was closed.
>> in 2012 , tamerlan traveled to the russian republic of dagestan for six months, investigators are scrutinizing tamerlan 's time abroad to see if he linked up with an terrorist organization . and just yesterday, the fbi interviewed his parents in russia .
>> whether he visited with someone there, if that led to his radicalization, it's one of the big questions in the case that the fbi is following up on.
>> tamerlan sporadically attended a local mosque in cambridge and exhibited odd behavior according to a senior member at the islamic society of boston .
>> he was obviously, he did not know the etiquette of handling himself or the etiquette of being part of the congregation.
>> tamerlan stood out after interrupting two friday prayers because he thought the preacher's teachings were not in line with islam. you don't stand up and interrupt.
>> even if you disagree?
>> would you say he had a strict interpretation of the koran?
>> i would say he didn't have a good understanding of the koran.
>> is sal and fellow members of the mosque are as outraged as anyone about the attacks. do you ask how and where he became radicalized?
>> terrorism analyst evan kuhlman doesn't believe he was a member an extremist organization. he was self-radicalized.
>> they take it upon themselves to carry out what they see as the ideology of al qaeda and similar groups and carry out attacks in the name of those groups, even though they may never have actually had contact with anyone inside of al qaeda .
>> tamerlan 's online footprint seems to indicate he was sympathetic to radical causes. he posted this al qaeda propaganda video on youtube. as for dzhokhar, his extensive tweets appear to show a typical american college kid. but in the wake of the bombings, others hint at something darker. investigators continue to check all angles, mine cell phone and computer records and also met with tamerlan 's american-born wife, who says she knew nothing in advance about the attacks. investigators have told nbc news that the brothers acted alone.
>> these are people who can learn exactly what they need to do entirely over the internet.
>> and investigators say dzhokhar told them that's how they learned to make the bombs. from "inspire" magazine, an al qaeda online publication in english.
>> a few years back, al qaeda in the arabian peninsula published deta detailed, specific instructions about how to build exactly such a device at home.
>> one of the components of the home made bombs is fireworks, which records show tamerlan bought in february at a store in new hampshire. and according to the criminal complaint against dzhokhar, the fbi seized from his room, a large pyro tech knick.
>> it appears that dzhokhar tsavraev told investigators they never rehearsed this. this was basically the only time they ever set off bombs. if that's true, that's quite extraordinary. they have three bombs, two at the marathon, one on thursday night. all three went off. they were 3-3. if it was the first time they ever built a bomb, quite extraordinary.
>> federal authorities not entirely convinced they didn't rehearse.
>> so typical in these cases that someone tries to rehearse this, practices to make sure they get it right, they want to check that out.
>> home made bombs, online instructions, attacks in our own backyard. those charged with protecting the public are concerned that this new type of attack will become terrorism's new normal. is this more frightening than two individuals could do something like this on their own, be off the radar?
>> the idea of lone wolfs, the idea of individuals who activate themselves and take the information that is easily available on the internet, and turn that into an attack is something that we've talked about since 9/11.
>> and that is the saddest part of all, that al qaeda is trying to promote the idea among these kids, these young deluded kids that they can become a celebrity, that this sort of thing makes them a hero.
>> as those who are killed in the attacks are mourned and dozens more wounded remain hospitalized, the kunlt hcountry has had a wakeup call we're still is vulnerable.
>> the whole nation is resting more easily since 9/11, feeling the threat may had abated somewhat. it clearly hasn't.
>> by striking a marathon like this, you're striking at america. killing americans celebrating the freedoms we have in this country. and these folks are wanting to make us live in fear and that's what they succeeded in doing in boston at least for several days.
>> but not on this day. this morning, boylston street , the site of the bombings, reopen reopened, back in business.