Dateline   |  April 24, 2013

The Hunt for Answers, Part 1

Explosions rock the Boston Marathon and with the public's help, investigators begin the hunt for the bombers.

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This content comes from Closed Captioning that was broadcast along with this program.

"terror in boston: 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 reportedly 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 of the two suspects, suspect one and suspect two.

>> five hours after the images flashed around the world, the suspected bombers came out shooting.