investigation about who did it and why. and in a scene that turned average citizens into first responders, the story of a bright spring day that ended in heart break and heroism. a special edition of

from boston, this is a special edition of "nightline," terror at the boston marathon with terry moran. >> good evening and thanks for joining us tonight from boston. a city reeling tonight in shock and sadness, really. we are just a few blocks away just down there from what was the finish line of the boston marathon. that magnificent sporting event. and in most years on this night these streets the heart of downtown boston would be humming with activity packed with people celebrating the achievement of the runners.

their loved ones coming from all over the world to support them. but tonight these streets are frozen in time that moment this afternoon when two bombs went off at the finish line. wounding more than 140, killing three, at least. including an 8-year-old. and tonight, the investigation is in full swing. the fbi is in charge. they are pursuing all leads, all of this, this grim day, a reminder of our country's constant vulnerability to sudden and murderous attack. the 117th boston marathon, one of the world's great sporting events. more than 23,000 started the race. half a million spectators along the 26.2 mile course. boston's is one of the world's six major marathons. runners from more than 90 countries. the winner crosses the finish

line at 12:10. then at 2:50 p.m., more than half the runners were through. the first bomb explodes just a few yards from the finish line. in the chaos, listen for the second explosion just moments later. another angle of the second blast. as people looked down the street at the first. >> something just blew up -- run! go! >> multiple people down here okay? i don't know what the cause is. stand by. >> reporter: terror turning the boston marathon into a war zone. more than 100 people injured. the youngest victim an 8-year-old boy killed by one of the blasts. >> oh, god get out of the stands. get out of the stands. >> all of a sudden there was a loud noise and explosion, big puff of smoke and a few seconds

later further up the road away from the finish line a second explosion, a large puff of smoke. and chaos, everybody running everywhere. >> a lot of casualties coming back. saw one guy with his legs gone at the knees. shrapnel wounds and wounds to the side of the head. other things. not good. >> i had crossed the 26 mile marker and saw the first explosion happen and there was commotion. i saw fire and smoke. and i didn't know what it was and i saw a trash can explode and people started throwing down the barricade and running over each other. i ran in the other direction as fast as i could. >> very scary sight. probably one of the most scary incidents that i've ever been involved with. very loud. the ground shook. you could feel it going through you. >> and tonight we are hearing the harrowing stories of survival from those just feet from the blasts.

local hospitals quickly mobilized for a mass casualty event but the chaos continued at tufts medical center where 11 victims were taken. a bomb scare forced the evacuation of the emergency room. patients on stretchers were laid out in the atrium. >> what injuries are you seeing? >> they are almost all lower extremity injuries from the knee down, very severe blast injuries with open fractures, severe muscle injuries, nerve, artery injuries. >> i had a bad feeling when i saw the smoke and when i saw after hearing the noise and hearing the second one. it's the boston marathon. it's one of the world's marathons. it's at the finish line. i suspected it was terrorism. >> reporter: rebecca roach had just finished the race when the bombs went off. >> it's a moment of what should have been joy and -- turned into panic and just heart wrenching fear of the worst. >> reporter: she was okay but her mom and a friend there to support her were badly injured.

>> makes me feel quite guilty. >> guilty? >> yeah. they wouldn't have been there if it wasn't for me. >> reporter: rebecca ran every step of the 26.2 miles with her mom and dad in her heart and written on her legs. >> is there any way to appreciate what you did accomplish today? >> yeah. >> good. >> i -- it was a dream of mine for years and i finally accomplished it in the time i wanted to. >> reporter: the bombs went off 50 to 100 yards apart with a number of cameras capturing the blast the investigators have plenty of evidence to study, frame by frame, as they hunt for who is responsible. the bombs went off four hours after the start of the race the average marathon time for male runners is around 4:30 hours. it was ideal marathon weather. it was patriots day in boston.

but by evening police replaced the finish line with yellow police tape. on a map of boylston street, you can see just how close, less than a block away from where the runners finished the course in triumph and so many of their loved ones waited. >> people should be calm. >> reporter: two hours later investigators seemed to be trying to make sense of it all. >> we're recommending to people they stay home. if they're in hotels in the area they return to their rooms. we want to make sure we completely stabilize the situation. >> reporter: president obama was notified in minutes and while he was not ready to use the word "terrorist" he made this vow. >> we will find out who did this we'll find out why they did this. any responsible individuals, any responsible groups will feel the full weight of justice. make no mistake. we will get to the bottom of this. >> reporter: but new york congressman peter king is more willing to point firngers. >> obviously it's nothing give net yet but it has all the

earmarks of an al qaeda attack. it reminds me of what happened in times square in 2010. it could end up being someone else but it has all the indicators of being an al qaeda or al qaeda affiliated attack. >> reporter: for rebecca roach and all the runners who ran with full hearts and their loved ones who cheered them on it's so incomprehensible and so wrong. >> i just think it's horrendous. why? i don't understand. i can't find reason in it. >> reporter: so it was a day of he heartbreak here in boston and of heroism. bruce mendelson who is an army veteran was on the scene and responded. you were right at the finish line? >> i was in a post-race party at 667 boylston street on the third floor of a building. i was sitting on a couch. the first explosion knocked me off a couch on to the ground. i was with my brother who had just finished the marathon. i yelled at him to get all the

people back away from the windows. and -- >> and you went down. >> he got people away from the windows and back of the building. i ran downstairs and came out on to a sheet of glass shards. there were blood smears on the sidewalk, there were people injured with a lot of grieve yus lower body injuries. >> what did you do? >> i tried to render what medical assistance i could. i tried to help the boston pd which was on the scene in force immediately, i tried to help them establish a security cordon so there wouldn't be any tertiary or other explosions. >> sounds like instincts took over, your training. >> i guess i could thank army for that. i guess i could what anybody else would do, wrong place, right time. >> reporter: how did it effect you seeing that? >> this is terrible. i mean, this is terrible. i'm upset. i mean, normally on marathon night this place should be full with people, bars open, people

happy and festive. and this beautiful day blew up in a second, in a heartbeat. to see what we've seen today makes me so angry. i'm angry. >> reporter: and you were responding to people injured by the blast. what kind of injuries did you see? >> a lot of lower body injuries. it leads me to believe that the devices were at or near ground level. i was trying to help for 15 minutes then the law enforcement officers invited me to leave. they should have. this is not my bailiwick or area of expertise. i left. picked up my brother. we walked back across the bridge with thousands of other people. looked very much like 9/11. >> reporter: people are calling you a hero. >> they shouldn't. i did what anybody else should do in these circumstances. we are a resilient people. this is what we should do to respond. >> reporter: thanks very much. thanks for what you did. >> okay. thank you. >> reporter: we'll be right back. [ pacino ] the inches we need

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it's been more than 11 years since the 9/11 attacks shocked this country into the 21st century and since then, there have been more attempted attacks. the shoe bomber, the underwear bomber, the times square bomber. plots foiled and plots we never learned of. over the years as the country's homeland defenses hardened, the fear seemed to abate until today. and now the attention focuses on

those two issues, who did this, bring them to justice. with the latest on the investigation here's abc's pierre thomas. >> reporter: it is dramatic videos like this and this, captured at the scene of the horrific blast today that investigators will be combing through for clues of what happened this afternoon. two bombs exploding within seconds of each other and only 100 yards apart. the bombs were small portable devices that could have been carried in a backpack, easily concealed among the thousands of spectators and that's what makes this investigation so difficult. >> doesn't take a lot of expertise to make small, handheld, backpack-sized bombs. you can go on the internet at this moment and it will tell you the components, it will tell you about timers, it will tell you about using cell phones to detonate it as one device. >> reporter: authorities say there are no suspects but peter

king saying there is a person at a local hospital who has come under law enforcement scrutiny. >> i've heard several reports that there is a person who is in custody. he is being questioned and i guess the best way to describe it is a person of interest right now. >> reporter: the bombs caused terrible injuries and hospitals reported seeing shrapnel wounds. >> there's a lot of small metal debris. some people have asked already whether these were bbs or parts of bombs. i don't think we're able to say whether these were small bits of metal placed there intentionally or part of the environment. >> reporter: the shrapnel is just one of the clues investigators will be looking at. >> there are fragments in the victims and in the sidewalk and what they will be able to do is probably reconstruct the timer device, what was used and look at the components of the bomb. it will tell them if this is

somebody who put this together over the internet or is this a bombmaker? >> reporter: hundreds of state and federal investigators have descended on boston to figure out who could have built the bombs and set them off. >> federal and state and local law enforcement are coordinating closely. the fbi has taken charge of the investigation. >> reporter: this type of attack has long been lawful's worst fear. a soft target where suspects can easily blend in and inflict mass casualties. >> unfortunately these kinds of settings, whether it's a marathon, football, baseball, social event, concert, are attractive targets. we can't get around it. >> reporter: the boston marathon is a heavily policed event. more than 500 national guard troops were on scene this afternoon as well as members of the boston police and fire department. but that did not prevent this from happening. >> anyone can walk up to the sidelines on that stretch of boylston street. anybody can get off the subway

or bicycle in or walk down and walk right up to the lines where this bomb took place. it would be impossible to screen everybody along the 26-mile route of a marathon. >> reporter: cities around the nation on high alert. bomb-sniffing dogs patrolling the streets of chicago. airports in washington and new york stepping up their security. in new york the stepped-up police presence could be seen everywhere. police cars lined the streets in times square, which itself was the scene of an attempted bombing in 2010. that was only foiled when a local vendor alerted police about a suspicious car. today's attack is one of many targeting u.s. civilians. in 1993 terrorists targeted the world trade center killing six people. two years later, in 1995, was the oklahoma city bombing. 168 people killed in that attack. similar to today's attack in boston, the bombing of the atlanta olympics in 1996.

spectators gathered in a park when the attack took place. and the horrific events of 9/11. thousands killed. seared into memory. tonight, investigators are just starting to comb through the evidence, collecting the security camera video from the scene, searching for any clues about who could have done this and why. >> the fbi and boston police and state police will be going over all the videotapes from that entire neighborhood, looking to see if there was someone suspicious or if there was someone they recognize, doing facial recognition analysis and running facial software, to see who was there. >> reporter: for "nightline" i'm pierre thomas in washington. >> the investigative task ahead. thanks to pierre for that. next up we talk to a survivor, one of the runners knocked down at the finish line. he made it through this harrowing attack.

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within minutes of the bomb blasts at the finish line of the boston marathon just a few blocks back there, the pictures emerged. so many pictures from bystanders, spectators, even runners. one has emerged as a kind of definitive image. a 78-year-old marathoner knocked down by the force of the blast. but he finished the race and was thankfully uninjured. now abc's lindsay davis with the interviews, the survival stories behind a menacing and iconic picture.

>> reporter: it's an image that stays with you at the moment of the explosion, bill knocked to the ground surrounded by smoke just 15 feet shy of crossing the finish line of what would be his third boston marathon finish. >> i could see the finishing ramp. when i got within 15 feet of it a horrendous explosion occurred. i didn't know where at the time. >> reporter: could you see it or just hear it? >> i could hear it. >> reporter: he approached the finish line 3:50 after he started the race. right in the middle of the pack. he was feeling great until he unexpectedly collapsed. >> my body was crumbling and i was going down. boy, this is going to be the end, maybe this is going to be my last trip. i didn't have any idea what was going on. >> reporter: the 78-year-old then crossed the finish line and aided by volunteers he then walked six blocks back to his hotel, not sure what had just happened. >> i figured that was probably

some kind of a bomb went off. it was -- didn't know what to think. >> reporter: which way was the finish line? >> this way. the bomb was behind me. >> reporter: okay. >> i didn't think about any possibility of another bomb going off up by the finish line. and then after i was up and walking over, then the other bomb went off. it was loud. i mean, really loud. my whole eardrums when i got back here i could hardly hear anything. it was tremendous. >> reporter: it is the story of tragedy and triumph for bill, who far more than finishing a race, is just relieved tonight that he's here to talk about it. for "nightline" i'm lindsay davis in boston. >> thanks to lindsay. thank you for watching abc news. "world news now," "good morning america," tomorrow they'll have all the latest. we'll leave you with the m.i.t. building lit up like the america flag in solidarity. good night.