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... See More
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 -- with people celebrating. Celebrating the achievement of the runners their loved ones coming from all over the world to support them but tonight. -- street seemed frozen in time at that moment this afternoon when two bombs went off at that finish line wounding more than a 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 countries. Constant vulnerability to sudden. And murderous attack. The 117. Boston Marathon one of the world's great sporting event. More than 43000. Started the race for half a million spectators along the 26 point two mile -- Boston's as one of the world's six major -- runners for more than ninety countries the winner crosses the finish line at 1210. Then at 2:50 PM more than half the runners were through. The first bomb explodes just a few yards from the finish and in the chaos listened for the second explosion. Just moments later. Another angle of the second blast. As people look down the street the first. Good -- at. Terror turning to Boston Marathon into a kind of war -- more than a hundred people injured. The youngest victim an eight year old boy killed by one of the collapse. All of a sudden there was of loud noise an explosion -- big puff of smoke. Later -- are up there. In an ultimate chaos everybody running everywhere it's not quite a few. Casualties coming back I saw one guy with his legs gone at the knees and some -- ankles and -- -- Shrapnel wounds and people decide that ahead and other things it just not good. I was -- crossed the 26 mile marker I saw the first explosion happened and there are some commotion I saw fire and smoke. And I didn't know what it was the man. I've sought trash can explode and people started throwing on the barricaded and run over each other matches -- -- in other directions massacred. Very scary but we're probably one of the most scary incident -- that I've ever been involved -- Very loud it -- -- here -- good feeling going down for you. And tonight we're hearing that harrowing stories of survival from those who were just feet from the -- Local hospitals quickly mobilized for a mass casualty event but the chaos continued at tufts medical center where eleven victims were taken. A bomb scare forced the evacuation of the emergency room. Patients on stretchers were laid out in the atrium. What kind of injuries things. They are almost all lower extremity injuries. From the knee down. And they are very severe blast injuries with open fractures severe muscle injuries nerve artery injuries. -- had a bad feeling when I saw the smoke and -- After hearing noise in -- hearing the second one. The Boston Marathon it's one of the world's marathon at the finish line. I suspected it was terrorism Rebecca roach had just finished the race when the bombs went off it was a moment of what should have been joy. And turned into panic and just heart wrenching fear of the worst. She was okay but her mom and a friend who were there to support -- were badly injured makes me feel quite guilty. Guilty. Yeah. Doing nothing there -- wasn't for me. Rebecca ran every step of the 26 point two miles with her mom and dad in her heart. In Britain under Linux is there any way to appreciate what you. Did accomplish today. Yeah. Good -- it was a dream of mine for years. And I finally accomplished it and then time that I want to answer. The bombs went off just fifty to a hundred yards apart. With a number of cameras capturing the blast investigate. -- have plenty of evidence to study frame by frame as they hunt for who is responsible. The bombs went off about four hours after the start of the race the average marathon time for male runners is around for a half hours. It was a deal marathon whether this was patriots day in Boston. But by evening. Police had replaced the finish line with yellow police tape. On the 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 -- people should become even two hours later investigators seem to be trying to make sensible. We're recommending to people that they stay home that if there are hotels and -- that they return to their homes more mature and completely stabilize the situation. At the White House President Obama was notified within minutes and while he was not ready to use the word terrorist. Made this we will find out who did this we'll find out why they do us any responsible individuals. -- responsible groups. Won't feel the full weight. The justice. Make no mistake. We will get to the bottom this -- but tonight New York congressman Peter King is more willing to point fingers. Obviously is nothing definite yet but it has all the hallmarks remarks. Al-Qaeda type attack reminds me very much -- what happened when Times -- -- -- It's when can. Again it could end up being someone else but right now only indicators. And every aspect of being yeah -- -- -- affiliated back. For Rebecca roach and all the runners who ran with full hearts and their loved one -- cheered them on it's also incomprehensible. So wrong and just think it's her and that's why. And understand. I can't find reason it. It's -- a day of heartbreak here in Boston and of heroism Bruce Mendelsohn who is an army veteran. Was right here on the scene and responding -- -- time we were you right. Back to finish the job I was at a post race party at 667. Boylston street. On the third floor of the building. The first explosion was sitting on the couch the first explosion knocked me off the couch onto the ground. I was at my brother who it who had just finished -- marathon and I yelled at him to get all the people back away from that windows. And -- you write down so he got -- the people back away from the windows towards the back of the out building -- ran downstairs to the first floor. -- came out onto a sheet of glass shards. There -- blood smears. On the sidewalk there are people. Injured with the a lot of greed is -- lower body injuries. And why did you do well I tried to render what medical assistance I could. I tried to help the Boston PD which is on the scene -- force immediately but try to help them establish a security -- so they wouldn't be any tertiary. Or other explosions. And again it's -- like your instincts. Took over -- your training. -- I guess I could think this united states army for that but I think I did it when anybody else would do no wrong wrong place right time. How -- it back to you. Seeing them as he came out this is terrible. I mean this is terrible I'm -- I'm -- -- set. And in the normally under marathon night there's just wish people would people bars are open people happy infested and -- this it's beautiful day. Blew up in -- second in a heartbeat and and to see what what we've seen today. It just makes me so angry I'm angry. And you were responding to people and injured by the blasts what kind of injuries did you -- I saw a lot of lower body injuries. I think we're here -- leads me to believe that that that the devices were at or near ground level. I was trying to help about fifteen minutes. Content that the law enforcement officers invited me to leave but I should have undergone this is not my -- it's not my area of expertise. I left. Picked up my brother and we walked back across the mass ave bridge with thousands of other people looks very much like 9/11. People are calling you -- to. Well and they shouldn't mean I didn't win anything I did what anybody else should should do in these circumstances we're resilient people -- this is what we should do respond.
This transcript has been automatically generated and may not be 100% accurate.