We were able to reconstruct it all moments and quite terrifying moment. Attack appears to -- Just before two feet -- PM this is the viewpoints -- by maybe 26000. Marathon runners we're approaching the... See More
We were able to reconstruct it all moments and quite terrifying moment. Attack appears to -- Just before two feet -- PM this is the viewpoints -- by maybe 26000. Marathon runners we're approaching the finish line. First of -- Rocks sidewalk along force white's book blasting. Metal barricades in the streets. -- crowds gathered. Just as the -- -- approaches before our intent. There -- about -- bill. Twelve seconds later as many -- to see him up at. A second explosion goes up. -- 130 yards in the first less than a block away. Very -- -- -- round here keeping it. He was going down for you. Injured lay on the pavement blood -- the ground. Emergency medical technician. Boston Police and some 400 national Guardsmen already on the race. Does the defense medical -- -- transform its promise. Saw one guy with his legs -- needs some -- Shrapnel wounds and people decide that ahead. Like 3 PM just as the president is being briefed on the attacks reports trickle -- of yet another explosion. This time at Boston's JFK library it's later. There are reports of additional devices several schools and hospitals -- -- Meanwhile over 130. Victims are being transported to six area hospitals just after 6 PM. President Obama addresses the country. Valley. To find those responsible and make no mistake we will get to the bottom of this any responsible individuals. Any responsible group's. Appeal -- -- The justice. Matter how many times you see it's still just as terrifying you want to show. A one photographing as well -- speaking. To what it was knots. The hundreds and hundreds of bags waiting for runners who were never able to claim them. Again a snapshot. Innocence and perfection on that patriots day. Suspended if not lives.
This transcript has been automatically generated and may not be 100% accurate.