. i didn't see them drop the backpack, and i was right next to it. >> we were rattled once they saw their faces on tv. >> they were confident. they were rapidly firing. what this photo of a victim and one of the suspects may reveal. what his photos may tell us about how prepared the brothers were, for a face-off with police. >> they had the perfect cover. >> they did. >> also, the portrait of a suspect takes shape, with

stories of tamerlan tsavraev's strange behavior in a music. >> he seemed to object. >> you just don't stand up and talk out during the service? >> you just don't. >> and an evolving picture of what else they may have been planning. >> they were going somewhere, and they had evil intent. >> is it possible that law enforcement, watchful witnesses, and technology came together to foil more attacks. was the boston marathon their last target? "terror in boston: the hunt for answers." welcome to "dateline," everyone. i'm lester holt on boylston street in boston. site of the marathon bombing. we have followed every development, but many of the biggest questions of the case have remained. tonight, we'll try to answer some of them. inside stories paint the most complete picture yet of how the bombing happened, the deadly manhunt that turned this city upside down.

who the suspects are, and what may have motivated them. and where this worldwide investigation goes from here. >> white male, dark hair. >> we know who they are. >> on our broadcast tonight, the suspects. >> we know what they have been accused of. >> over 100 people injured, some gravely. >> but questions remain. why did it happen? did they act alone? what else were they planning? >> they wanted to have that bomb marathon style used again. >> that subject is in custody. again, he is in custody at this time. >> tonight, new answers, new details about the marathon bombings and the violence that followed. how professional investigators put a stop to it. >> cease-fire! >> and how an army of citizen sleuths helped them do it. >> the slogan, see something, say something, now see something, say something, and take a picture of it. >> a new war on terrorism may be opening, with a different kind of enemy.

>> these are people who can learn exactly what they need to do, entirely over the internet. >> the fbi began working the marathon bombing case on monday, april 15th, hours after the attacks. ultimately, the agency built its case on images on a precisely choreographed attack. cameras show 19-year-old dzhokhar tsavraev and his older brother tamerlan on boyleston street. both carry backpacks. tamerlan plants the first bomb. dzhokhar walks to the forum restaurant and into this photo. there is someone else here too. jackie webb. when you see the photo now, it hard to believe it was right there? >> yeah, it is. >> how close to that were you, to where the bomb was placed? >> probably just five or six feet. >> that group of people was most at risk. >> um-hum. >> this photo captured a crucial moment, placing dzhokhar at the scene.

investigators say after putting the bomb at the site, he walks away, with just ten seconds to spare before the blast. in fact, the fbi released a cropped version of this picture in the hunt for the suspects. >> my boyfriend and his friend. >> this week, i spoke with jackie in her first interview since the bombings, she is 25 years old. a realtor in winchester, massachusetts. she was at the finish line with a group of friends, including her boyfriend, paul nordin. >> we were watching a friend of ours run, ex-marine and a firefighter. >> the first bomb went off a block away. >> at first, you kind of think, a gunshot, a canon, i turned around and one of my boyfriend's friends started screaming it was a bomb. we looked down boylston street and you could see the cloud,

like a mushroom cloud, going up. >> her boyfriend and her brother heaved her into the street just before the second bomb exploded. >> what do you remember about the second blast? >> it's a big white light. this sound that is incredible and i just remember it just being so dusty and just see smoke. >> her hands were burned and so was her stomach. one leg was splattered with shrapnel. the other had two deep holes. a piece of zipper was embedded in your leg? >> my surgeon came in the next day and showed me on the x-ray and said we took it out, sent it to the fbi. >> it was part of the bomb? >> yeah. part of the bomb or probably part of the backpack. >> jackie has more surgery ahead of her. but she knows she's luckier than some, including her boyfriend paul and his brother. each of them lost a leg. how do you process all of this,

that someone would attack this city and attack something as innocent as the boston marathon? >> i think it's disgusting. i don't know how you can possibly do this. i'm lucky to be alive, and they took the brunt of it. >> never saw anything suspicious beforehand, nothing. >> i didn't see the backpack, i didn't see them drop the backpack, right next to it. >> others did see suspicious activity. 27-year-old jeff bauman was seriously injured that day. when he woke up in the hospital, he is said to have written a note to investigators, saying bag, saw the guy. looked right at him. tips like this would come to define this investigation. >> citizens were really striving to be helpful to law enforcement. >> dan conley is the district attorney for boston. he helped launch the investigation into the biggest

terror attack on u.s. soil since 9/11. it was clear to the eyes the amount of human damage. >> i have been to many crime scenes, but never seen anything like it in my life. >> investigators began as they always do, by collecting evidence piece by piece. sifting, combing, probing for clues. but this time the clues weren't always on the ground. this time, they were embedded in images. >> the first big role and task of law enforcement was to literally go up and down boylston street, get in touch with owners of businesses and secure that close circuit television video evidence. >> a mind numbing task. one investigator recoe reported looked at a piece of video 400 times and then there are personal videos and photos. a torrent swapped, shared, and posted online by the smartphone generation. photos like the one that shows

jackie and dzhokhar. whether it was bauman's tip or something in the images, investigators zeroed in on these two men. brothers who emigrated to the u.s. a decade ago from a predominantly russian republican. tamerlan tsavraev, married with a child, 26. dzhokhar, six years longer, enrolled at umass boston. >> i would see him with his headphones on being going to class. >> the younger man chose to hide in plain sight after the bombings. two of his friends, who talked to nbc's jeff rossen said they met him at the gym the next day and they chatted about the only thing on everyone's mind. >> i talked to him that it was crazy we had a bombing in boston. >> you said that? talked about the bombing? >> yeah. i said it's a tragedy. he said it's a sad thing. >> he said it's sad. >> he looked me in the eyes.

>> i noticed he was looking over his shoulder. >> his car mechanic also saw him the day after the bombings. and that encounter seemed to indicate a jittery 19-year-old. >> just told him the car wasn't done. and he told me i need to pick up the car right now. >> still, the brothers might have remained under the radar if not for pivotal decision by the investigators. >> we are releasing suspephotos the two suspects, suspect one and suspect two. >> five hours after the images flashed around the world, the suspected bombers came out shooting. coming up, not even veteran cops could predict the horrifying way these wanted brothers would burst back onto the scene. >> my reaction was total disbeli disbelief. >> what could it tell them about the suspects they were chasing? when terror in boston, the hunt for answers, continues.

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with the fbi's release of photos and videos of the two suspected bombers, the 24-hour news cycle launched into high gear. the hunt was on. >> now we know who they are looking for. >> these are the suspects. >> who are these two men? >> their faces were plastered everywhere. a stream of tips pouring in. and the walls began closing in on the suspected bombers. >> once we saw their picture show up on tv, they went into panic mode. >> nbc news terrorism analyst

aaron kuhlman believes the suspect's blown cover thrown them into a tailspin. while the bomb was executed with precision, now it appears they have no plan of escape. >> they started making grievous errors, in retrospect, i don't think you could possibly explain. i don't think they were the product of rational thinking. >> as darkness fell on boston last thursday, the entire region was still on edge. no one knew where the suspects were, what they were planning next, or when they would resurface. m.i.t. campus police officer sean collier was on patrol in cambridge around 9:30 p.m. when his boss, john difava spotted him. >> i pulled up next to him in the cruiser. i said what are you doing? just watching traffic, chief. keeping my eye on things. >> one hour later, the chief got the call, officer down. collier had been fatally shot in his patrol car. >> my reaction was total disbelieve.

i know i have 57 brokenhearted police officers. we have his family who is devastated and a community that, you know, obviously from the outpouring of sympathy and support, they are devastated too. >> why ambush collier? the shooting continues to confuse and baffle cambridge police commissioner robert hawes, who gave "dateline" a detailed account. >> it appears somebody or somebodies approached from behind collier's vehicle and fired four or five rounds into the vehicle, striking officer collier each and every time for the number of shots we could determine. >> did the suspects think officer collier had recognized them? or perhaps even though they had at least one gun, they wanted his weapon too? boston police commissioner ed davis. >> why ded they pop up back on the radar? why did they kill officer collier?

>> it is hard to say. it goes to part of the investigation that is impossible for me to talk about. for some reason they activated themselves, started to kill again, and they were heavily armed, so i think people have to draw their own conclusions at this point in time as to why they did that. >> and the violence wasn't over yet. midnight when police responding to cambridge, there was a carjacking not far away. >> too many things in close proximity for this to be coincidences. >> it was the bombing suspects and they took their carjacking victim on a terrifying ride. he later told investigators that one of the carjackers pointed a gun at him and said did you hear about the boston explosion? and i did that. the suspects also demanded cash, and drove the victim to three different atms. here is a surveillance photo of dzhokhar at one of them. the next stop, and what appeared

to be an improvised flight was this gas station. >> they pulled into a gas station to refuel, his car was about out of gas, he saw his chance and escaped. >> justice correspondent pete williams has been leading coverage of the investigation. he communicated with the carjacking victim via e-mail earlier this week. >> he described them as brutal, but cautious and said he was in fear for his life. >> the carjacking victim escaped by running from the gas station to another station across the street. and asked attendant tareq ahmed no help. >> he said, please, please call the police. someone wants to shoot me. they have the bombs, they have guns. they took my car. they -- they want to kill me. >> the suspect's plan, whatever it was, was unraveling. the calm demeanor known by authorities in the surveillance video, seemed to be replaced

with panic and desperation. >> they acted in a very impetuous and irrational manner, but that may also indicate that they didn't know what to do with that point. their plan, their plan had come to an end and they didn't know what their next step would be. >> the carjacking would prove to be their fatal mistake, undone by police with help from a cell phone. a citizen, and modern technology. coming up, a resident turned reporter captures important images of the virtual war zone out his window. >> i could feel the explosions in our apartment. >> and as the fire fight escalates, investigators learn something new about the suspects. when "dateline" continues.

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>> . >> just west of boston, the watertown police department was on high alert after the brazen killing of the m.i.t. police officer and the carjacking of the mercedes suv. >> we had a wild late-night, early morning. >> but police chief edward deveaux, never expected his town to become the final battleground

for the two suspects wanted in the boston marathon bombing. >> pretty much 24 hours straight. >> the police chief, and ordinary citizens who documented the fiery showdown gave "dateline" chilling insight into how determined and desperate the two men were. >> i don't know if there are too many police departments in the country could say that they were in the fire fight we were in early friday morning. >> a fire fight that followed a lucky break from authorities. the stolen suv still had the owner's cell phone inside it. >> and we were able to ping the phone and kind of pick up where its location was. >> police also put out an apb for the vehicle. at some point the brother has picked up a second car. at around 1:00 a.m., technology and police work came together. a watertown officer saw the stolen suv. >> and he started to follow them, as it turned out it wasn't one vehicle, it was two vehicles.

the two brothers were in two different cars, driving pretty much in tandem. >> as the watertown officer tailed them, both suspect cars suddenly stopped. the drivers got out and never hesitated. they fired directly at the officer, who reacted just as fast. >> he was able to get in reverse and get out of harm's way a little bit and put some distance between them as he was being rained on with bullets. >> six additional watertown police officers, three of them offduty, raced to the scene. within minutes, sleepy laurel street was transformed into a combat zone. >> sounded like gunfire, and i immediately got up and went to our window and looked outside. >> by now, you may recognize andrew kitzenberg. he became a citizen reporter that morning as he watched the unfolding battle from his window, he caught these amazing images with an every-day smartphone, capturing the

tsavraev brothers shooting at police from behind the stolen suv. >> andrew, would you be able to show us that? do you feel walking your computer to the window? >> yeah, i'd be happy to. >> after the gunfire, he was on msnbc via skooip, tuype, turnin laptop to a camera out of the window below. >> i came to this window. >> what he showed "dateline" documented how freshs terocious confrontation was. >> directly outside were the two shooters behind their cars, and the gunfire continued. >> describe these -- these two men at this point. they are firing guns. does it look like they are organized? >> they look quite organized. there were two vehicles. one -- one vehicle was the black mercedes suv was facing against the police officers, the two shooters were taking coverage

behind that vehicle and with the second vehicle, the green sedan, they actually had their back door open? >> they had the perfect cover. >> they did. >> christian tunsing is andrew's roommate. when you saw these guys, did they look comfortable in that they knew how to use these weapons? >> they were confident. they were rapidly reloading handguns, a constant barrage. >> bullets flying everywhere on laurel street, into the sides of houses and cars. >> the bullet hit the outside wall. >> one tore through a wall and chair where christian had been sitting earlier that evening. even through the pounding bombardment, andrew could hear the shooters themselves. were they talking to each other, yelling anything? >> yes, they were. i couldn't hear it too well, but you could hear dialog and i could hear yelling from my room. >> for a while, the two young shooters, despite having had no

weapons training that we know of, held their own against the police. >> they hung in there in this very small area and we're seeing 200 to 300 bullets were exchanged there. >> it terrified residents throughout the neighborhood and many took to social media, sharing information and fears about the gunfire over on laurel street. rebecca krieger, who lives about a mile away, was one of them. >> it was really scary. my family was asleep. i was awake kind of freaking out. i woke them up. >> she and her parents huddled in a second floor bedroom, watching for news. >> we had three different computers, all watching different news stations and we had, you know, all of us on our different twitter accounts and it was scary. we were kind of freaking out. >> back on laurel street, andrew kitzenberg and his roommate were trying to avoid the bullets. that's when andrew heard and felt the blasts. >> i could feel the explosions. i could feel it in our

apartment. >> at what point in all of this did it dawn on you, these have got to be the bombers? >> explosives. the explosives brought it to a whole other level. i saw a metal device that looked like a pressure cooker and definitely at that point, i knew what we were dealing with. >> when that one exploded, it rocked the whole place. to the police chief, it sounded all too familiar. >> it just, the exact duplicate of the bomb that went off at the boston marathon. on a side street in watertown. >> his men weren't just being shot at, but they were being bombed to counterterrorism experts, that signals a certain level of sophistication. >> despite the fact that you can get plans on the internet, to pull it off successfully generally means you have some type of training. >> former fbi agent don burelli. >> it does take training and practice, and most who are successful bombers do have training and have practiced

before they actually go and try to do the, you know, the event. why, does it work? >> the brothers, outnumbered by police, were ready to make their boldest move yet down laurel street in a revealing show of force and firepower, no one here would forget. coming up, one suspect makes a fateful choice. >> running down the street, charging and still engaging in gunfire. >> running into the face of this police fire? >> yep. >> what investigators say that moment may have revealed about the brother's relationship. when terror in boston, the hunt for answers, continues.

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move, move, move! >> around 1:00 a.m. friday morning, watertown was an armed camp with police, the fbi, and national guard on the streets. they responded there after police had engaged in a ferocious fire fight with the suspects on a tree-lined suburban block called laurel street. >> about ten or 15 feet from where the two shooters were. >> from his bedroom window, eyewitness andrew kitzenberg had a front row seat as bullets cracked in the night. his photos and observations offer some of the best information we have about how the suspects behaved. what he saw suggests the bombing suspects had absolutely no fear of death. kitzenberg describes a key moment when one of the brothers detonated the chilling explosive device. >> it created a huge cloud of smoke. at least 50 feet high and covered our entire street. >> what the elder brother did next reinforces the idea that he

was the lead err er of what investigators call a two-man terrorist team. under the cover of smoke, tamerlan ran toward the cops, arm extended, clutching a weapon, firing round after round. >> running down the street charging and still engaging in gunfire. >> running into the face of police fire? >> yep. >> no cover? >> no cover. he stayed toward the sidewalk, toward the side of the street on the fenceline and ran right toward them. >> firing? >> yes, guns ablazing. >> it sounds like a movie. >> it absolutely felt like something from a movie. >> those who knew tsavraev, like family friend luis vasquez. this fit right in on their dynamics. >> i think little brother was a follower. i think his younger brother was weaker minded. no way he could have been a master brain like a plan like this.

>> that night on laurel street, the older brother's blind charge turned into a face-to-face shoot-out. police chief edward deveaux. >> no less than five to ten feet away from each other. >> at some point, tamerlan was wounded, and in the midst of the vicious fire fight, the cops caught a break. >> he runs -- the bad guy runs out of ammunition. thank god. we tackle him in the middle of the street. we know he's shot. but he's still alive. very alive. and we tackle him in the street, try to get him handcuffed. >> as the officers were busy subduing him, witnesses say his brother dzhokhar jumped into the hijacked suv and made a bizarre move. instead of gunning it away from police, he turned around and headed straight for them. >> he does a u-turn? >> yep. he got into the vehicle and did a u-turn right in front of our apartment, and just started accelerating immediately. >> right toward the police?

>> yep. right toward the police. >> was he trying to help free his brother from police or just plain panic? at that point, an officer looked up, saw the hijacked suv barrelling toward them. >> somebody yells get out of the way. they dove, barely get out of the way. >> that's when dzhokhar ran over his older brother. >> did you see it go over him? >> at that point, i couldn't tell whether it went over him or not. i wasn't aware of it. >> how did he get through the watertown police vehicles? >> speed. he was going very fast, and he actually -- went in between them, sideswiped both of them. took out windows and doors on both cars and at least two, could have been a few more. there were a number of vehicles down there. >> the younger brother drove off into the night, police losing him on the dark streets of watertown. somehow, he had evaded the group of armed police officers.

police now had a suspect on the loose in a stolen suv. the other suspect critically wounded was on his way to a trauma center. there, an extraordinary encounter was about to take place. dr. david showenfeld in his home a few blocks away. an e.r. doctor at best israel hospital and had treated victims at the marathon bombing earlier in the week. as he listened to explosions and gunfire in his own neighborhood, he knew there might again be victims on the way to his hospital. >> i expected after monday that there was going to be a lot of people coming in. and so i called them and said start getting the department ready, and i woke my wife up, told her i had to go to work. told her i loved her and i would be okay. and i ran out of the house. >> the first patient to arrive had suffered injuries from head to toe.

>> it was a patient who was in critical condition that we had to save, and that was what we were focused on. and then when the patient ultimately was pronounced dead, and we stopped what we were doing, i think is when, you know, it sort of sank in. >> the man his team tried to save was one of the suspects who had been shooting up his neighborhood. >> it was, hey, this is -- this is the guy. >> tamerlan tsavraev. the man in the black cap from the fbi photos. none of the watertown police officers was injured. but a boston transit officer, richard donohue, who came to help his fellow officers, was seriously wounded. now, the most intense manhunt in the history of boston was under way to track down the younger brother who escaped. coming up, a resident has a ring side seat as the most wanted man in america is captured.

>> they are looking out the window, and that was when we knew. they got him, it's all over. >> but just who was this young man they caught? later, some answers. when "dateline" continues. because now i get to have my first love and my greek passion together, what i call a healthy marriage. activia greek. the feel good greek. ♪ dannon geico and we could help youo save on boat and motorcycle insurance too. other insurance companies are green with envy. oh, no, no, no...i'm sorry, but this is all wrong? i would never say that. writer: well what would you say? gecko: well i'd probably emphasize the savings. ya know...lose that green with envy bit. rubbish. it's just a reference about my complexion. writer: but the focus groups thought that the...

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with a possibly armed fugitive on the loose, watertown police took unprecedented measures last friday and were forced to make extraordinary decisions. >> you got to get out of here! >> regarding public safety. >> move, move, move! >> it's important folks remain indoors. >> after a night of terrifyi ii gunfire, the governor announced a lockdown of watertown and nearby towns and then expanded it to all of boston. the transit system was shut down, schools and businesses closed. as authorities ordered more than a million people to stay indoors. all of this, to find one man. >> i was scared. it was kind of surreal. i didn't really believe this was happening in my town and, you know, i couldn't really figure out what was going on.

>> law enforcement flooded watertown, and zeroed in on a 20-block zone to search for dzhokhar tsavraev. at times it seemed the only people on the streets were members of the media. >> i was standing here all day while s.w.a.t. teams were going house to house. in your gut, did you think you would find him in this town? >> after we pinned him down, i kind of did. the 20-block perimeter. we were fairly confident he was inside that. >> but where? morning turned to afternoon with no sign of the boston suspect, had he managed to hop a train before amtrak service was shut down? and then after 6:00 p.m., a surprise announcement. one that seemed more curious than calming. >> the stay indoors request is lifted. the t is open effective immediately. transf .

>> it raised a big question. why did authorities allow people to leave their homes as night was falling? had they given up, or was it a tactical move? boston police commissioner ed davis. on friday the search was going nowhere, but a killer still on the loose. were you hesitant about lifting the knockdown? >> we hesitant about everything we've done, making the best decisions we could. i can't get into details as to why we decided to lift it, but we did. thankfully. when the citizens started to move around, we found the suspect. >> they found him only after one of the average citizens cracked the case. just less than an hour after the lockdown was lifted, one watertown resident stepped into his backyard. and saw something strange on his boat. he described the scene to his stepson.

>> small pool of blood, as if someone had either -- i say someone. you know, being -- putting myself in his mind, i would have thought, hey, i haven't had this boat open all winter. did an animal crawl in and die? is it a squirrel? >> it wasn't an animal, it was a human, it was taken overhead. it touched off the next wave of chaos. rebecca krieger so frightened during the night during the shoot-out, was now a few houses down from the center of the action. >> it felt a lot like a war zone in watertown. >> she took to social media to tell the world what she was seeing with photos, showing the havoc one young man has called in his town. >> he is moving. we have movement in the boat. >> police brought in an fbi negotiator to talk to dzhokhar tsavraev. who was lying in that boat,

wounded. finally, he surrendered. and was placed in an ambulance. >> we saw the ambulance drive away. and that was when we knew and we started -- you know, phones blowing up again, they got him, it's all over. >> that subject is in custody. he is in custody at this time. >> but even in the euphoria of that night, law enforcement officials had come to a frightened conclusion, the bombers had other targets in mind. the evidence? the bombs the suspects had deployed during the fire fight. >> my gut tells me, yes. they had another bomb, they weren't saving that for the watertown police. they were saving that to do another event similar to the boston marathon. >> your officers may have stopped another attack. >> i strongly believe that, yes. they had weapons and bombs and were going to use them. >> was the boston marathon their last target? >> i don't think it was. they were going somewhere and had evil intent.

>> if a plan for regular bombings had been foiled, regular civilians played a role in stopping it. to the tips called in, the photos, and the man who went in his backyard and bumped into an alleged bomber. the fbi can get closer to answering the big questions. what drew the tsavraev brothers to this attack? had they acted alone? coming up from a local mosque, a story about tamerlan, odd behavior during worship. >> he he seemed to object. >> what did it mean? and did the brother's story tell us there is a new normal in terrorism?

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after dzhokhar tsavraev's capture, investigators 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. >> no. >> even if you disagree? >> no. >> 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? >> yes. >> 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 hcountr 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, reo n reopened, back in business. >> that's all for this edition of "dateline." we're off next wednesday. we'll see you next friday at

9:00, 8:00 central. i'm lester holt for all of us at nbc news, good night. . - in the criminal justice system, sexually based offenses are considered especially heinous. in new york city, the dedicated detectives who investigate these vicious felonies are members of an elite squad known as the special victims unit. these are their stories. - you look nice. - aw, thanks. - friday night date with cassidy? - uh, yeah, sort of. we're taking his mom out to an early bird dinner. he'll make it up to me later. - his mom? you two getting serious? - well, i wouldn't say we're picking out china patterns, but you know, it's a nice fit. - good for you. - thanks. you up to anything? - um, no. actually, tonight i'll be at home picking out china patterns. [elevator dings] i'm still refurnishing my cleaned-out apartment. besides, my neighborhood friday night, bunch of college kids getting drunk.