Within minutes of the bomb blast 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... See More
Within minutes of the bomb blast 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 78 year old marathoner bill differed knocked down. By the force of the blast but he finished the race and he was thankfully uninjured. Now ABC's Lindsey Davis. With the -- the survival story behind a menacing and iconic picture. It's an image that stays with you. The movement of the explosion feel it's -- not to the ground surrounded by smoke just fifteen feet shy of crossing the finish line. His third Boston Marathon finish. I concede the finishing rampant and were -- -- within about fifteen feet and list as horrendous explosion occurred. I don't know -- at the time just could you. It is here I -- here he approached the finish line three hours and fifteen minutes after -- started the race right in the middle of the pack he was feeling great. Until -- unexpectedly collapsed. Nobody was just. -- -- and I was going down nor there is internally and is going to be my last trip and and then there was going on. This 78 year old then crossed the finish line aided by volunteers. He then walked six blocks back to his hotel not sure what had just happened. And I figured that was probably some kind of -- bomb went off. So -- actually was that finish slightly. Usually yes this yeah finish line is this the -- was -- behind me. The I didn't think about any possibility of another bomb going off by the -- the finish line. And then after -- was -- and -- walking -- than any other bomb went off. It was loud I mean really not my -- -- where I come back here I could heard hear anything there was tremendous. It's a story of tragedy and triumph for bill -- far more than finishing -- race is just relieved tonight that he's here to talk about it. For Nightline I'm Lindsey Davis in Boston.
This transcript has been automatically generated and may not be 100% accurate.