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♪ it's sacrilege sacrilege sac-ri-lege you say ♪ tonight on "nightline," act of terror. two bombs explode causing carnage near the finish line of the boston marathon. over 130 wounded and at least three dead including an eight-year-old. we are live with the details. with law enforcement on high alert the unfolding 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 "nightline," terror at the boston marathon is 60 seconds away. get some friends, loser! so, are you all right, man? ♪ lean on me, thanks.

>> 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. 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 when two bombs went off at the finish line,

wounding more than 140, killing three, at least. including an eight-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 90 countries. the winner crosses the finish line at 12:10 and 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! >> 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 eight-year-old boy killed by one of the blasts. >> oh, god 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 loud puff of smoke

and chaos, everybody running everywhere. >> saw a few casualties. one guy with his legs gone at the knees. shrapnel wounds and wounds to the side of the head. 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. >> a scary sight. one of the most scary incidents i have been involved with. the ground shook. you could feel it going through you. >> and tonight we are hearing the stories of survival from those feet from the blasts. local hospitals quickly mobilized for a mass casualty event but the chaos continued at tufts medical center a bomb

scare forced the evacuation of the emergency room. the patients were laid out in the atrium. >> what injuries are you seeing? >> lower extremity injuries with open fractures, severe muscle and nerve 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. >> 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 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 time for male runners is 4:30 hours. it was patriots day in boston. but by evening police replaced the finish line with yellow

police tape. let's that a block from the finish line where many loved ones waited. two hours later, investigators seemed to be trying to make sense of it all. >> we recommend that people stay home. if they are in hotels in the area, return to their rooms. we want to 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 and 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 willing to point fingers. >> 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. >> why? i don't understand. i can't find reason in it. >> reporter: so it was a day of heart break here in hostage 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. 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 to the 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 a lot of people with grievous lower body injuries. >> what did you do? >> i rendered what assistance i could. i helped the boston pd establish a security cordon to make sure there would not be any other explosions. >> sounds like your training took over. >> i think i did 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 should be full with people and bars open and festive and it blew up in a second, in a heart beat. to see what we've seen today

makes me so angry. >> reporter: and you were responded to people injured by the blast. what 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 officer invited me to leave as they should have. this is not my baliwick or area of expertise. we walked back across the bridge. >> reporter: people are calling you a hero. >> they shouldn't. i did what anybody should do in these circumstances. we are a resilient people. >> reporter: thanks very much. thanks for what you did. >> okay. thank you. >> reporter: we'll be right back. [ pacino ] the inches we need are everywhere around us.

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it's been more than 11 years since the 9/11 attacked shocked this country and since then there has been the shoe bomber, the underwear bomber, the times square bomber. plots foiled and plots we never learned of. 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: is it 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 so difficult. >> it's not a lot of expertise. a small backpack sized bombs you can go on the internet and it will tell you the components and tell you about timers and using cell phones to detonate it as one device. authorities say there are no suspects but peter king saying there is a person at a local hospital who has come under

scrutiny. >> i have heard there is someone in custody. he is being questioned and i guess the best way to describe it is a person of interest right now. >> reporter: hospitals reported seeing shrapnel wounds. >> a it will of small metal debris. some have asked if they were bbs or parts of bombs. i can't say whether it was placed there or part of the environment. >> reporter: the shrapnel is just one of the clues. >> there are fragments in the victimance in the salk 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 someone put it together over the internet or is this a bomb maker. >> reporter: hundreds of state and federal investigators have descended on boston to figure out who 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 been law enforcement's worst fear. a soft target where suspects can blend in and inflict mass casualties. >> unfortunately these kinds of settings, a marathon, a concert, they are attractive targets. >> 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 walk down 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 on the streets of chicago. airports in washington and new york stepping up their security. in new york the police presence could be seen everywhere. police cars in times square which was the scene of an attempted bombing in 2010 that was foiled when a local vendor alerted police about a suspicious car. tonight the attack is one of many targeting u.s. civilians. in 1993 terrorists targeted the world trade center killing six people. 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 attacks took place and the horrific events of 9/11,

thousands killed. seared into memory. police are collecting security camera video from the scene searching for clues about who could have done this and why. >> the fbi and boston police will be going over all the videotapes from that entire neighborhood looking to see if there was someone suspicious or 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 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. ♪ [ male announcer ] how do you engineer

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within minutes of the bomb blast at the finish line of the boston marathon the pictures emerged so many pictures from bystanders, spectators and runners. one has emerged as a definitive image. 78-year-old marathoner, knocked down by the force of the blast. but he finished the race and was thankfully uninjured. now lindsay davis with the interview, the survival story 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 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. >> could you see it? >> i could hear it. >> reporter: he approached the finish line 3:50 after he started the race. he was feeling great until he unexpectedly collapsed. >> my body was crumbling and i thought this is going to be the end. i didn't have any idea what was going on. >> reporter: the 78-year-old then crossed the finish line and then walked six blocks back to his hotel not sure what had just happened. >> i figured a bomb went off. it was -- didn't know what to think. >> which way was the finish line? >> this