Roughly six hours before -- -- terrifying blasts on Monday. All eyes were focused -- -- Hopkinton Massachusetts the starting line of the legendary Boston Marathon. 126 point two mile course that winds... See More
Roughly six hours before -- -- terrifying blasts on Monday. All eyes were focused -- -- Hopkinton Massachusetts the starting line of the legendary Boston Marathon. 126 point two mile course that winds its way through leafy suburbs before reaching -- Copley Square. Sun -- cloud cover fifty degrees perfect day for America. Shortly after 9 AM to. Just over 23000. Runners begin their journey. And 58 AM. The winners crossed the finish line. Over the next few hours thousands more exhausted and triumphant do the same. 2:37 PM. Security cameras captured these images of two -- snaking their way through that -- crowd gathered along Boylston street. One in a black -- tem -- Sinai at. The other in white his younger brother Joseph -- Dual explosions 550 feet apart. The first hear the second here. And at. An instant chaos and carnage spectators and runners in both locations not to the ground. Instantly it's it was the brightest light and allowed us -- in your -- and everybody started running. They're making it very -- it to the ground -- you can just feel -- going down three of. As the smoke clears cameras from all angles -- what looks like a war zone sidewalks painted in blood. Victims paralyzed in shock and confusion. It's not quite a few casualties coming back I saw one guy -- legs -- at the knees and suddenly ankles and feet -- Shrapnel wounds and people decide that ahead. This smoke cloud and that kinds of injuries that you heard about in the beginning tell you. About when it happened. When we saw the film and saw that it was all white cloud. That -- it wasn't a heavy duty explosive. To admit it was -- space smaller more improvise more amateur device. Just after the blast David Greene takes this photo that would later prove critical. I immediately ever analyze the people I thought out of my -- and exactly -- picture. As -- -- -- there are others who run toward the epicenter. Police marathon volunteers even good samaritans like Michael chase scene carrying a little girl named Jane -- I was -- -- -- to help people to be honest with the. So was cowboy hat wearing Carlos Arredondo. Captured Wheeling badly wounded 27 year old Jeffrey -- now an iconic image of this tragedy. First -- Plus we just -- -- street -- stop helping now the people. Law enforcement filled the now evacuated area and secure a vast and intricate crime scene. The injured are rushed to area hospitals where doctors like George no -- -- spring into. Action had to do for occupations. Quite frankly. They were. There were almost completely amputated it's by the bomb blast. At 4:12 PM reports of another blast near JFK library in Dorchester later revealed to be an electrical fire. Did it spreads cities in the northeast catch the same case of the jitters New York City and the capital go on high alert. 6:10 PM with Pennsylvania Avenue closed to pedestrians President Obama offers condolences to victims in Boston. And we will find out who did this we'll find out why they -- this any response any responsible individuals. -- responsible groups. Will feel the full weight. Of justice. Tuesday begins with more Harding 176. Confirmed injured and -- -- till now at three. Including an eight year old -- returns and turns out he's the older brother and change seen -- from the scene by Michael chase. 9:30 AM Massachusetts Governor Deval Patrick an FBI officials appealed to the public for any photos or video the hands of the marathon. And no deet -- is considered too small. We'll go to the ends of the earth to identify the subject or subjects were responsible for this. To despicable crime. 11:30 AM President Obama speaks to the country again this time using the word everyone is waiting to hear the FBI's investigating it as an act of terrorism -- Meantime Joseph -- -- -- -- in his state of agitation. Goes to his mechanics auto shop to pick up a white Mercedes. Very agitated and obviously thought he was very polite -- -- is not -- community use. You couldn't -- know that you -- we don't notice. That the killers valuable. Person. Photos now appear online of what's believed to be components of the bombs. A -- pressure cooker. And it tattered black backpack there are also photos the circuit board and battery quickly linked to the distributor of the popular children's remote controlled car -- 2 PM 29 year old crystal Marie Campbell becomes the second victim identified. We have -- At the jets and that's not christened -- This -- -- strengths. Some families of the victims grieve and -- -- and nine and later tweets. Nowadays everybody wanted to -- -- got something to say but nothing comes out when they -- their lives just a bunch of gibberish. Wednesday 9:30 AM. -- 200 tents plastic UMass Dartmouth word spreads that authorities are closing in on a suspect this is a fast breaking investigation with a major breakthrough already. And the Internet is on fire with speculation. Web sites like four chant and read it and meticulously dissecting photos of -- -- Raising suspicions about faces in the crown. All including this young man turn out to be not in bombs. If you look at -- credit there was an eight. 1:30 PM. Wolf we have information and arrest has been made CNN. News reports an arrest has been made and will be announced shortly -- BI but that press conference that never happens because a bomb threat empties the federal courthouse. The final victim is later identified as Lou Lindsey a Chinese national attending grad school at Boston University. Wednesday -- -- with yet another tweet from Joseph cars and diet saying -- -- stress free kind of guy and. Thursday morning with the bombers still on the loose the president and First Lady arrive in Boston for an interfaith service in honor of the victims. Your -- Koreas -- view we will all be with you as you learn to stand. And walk. And yes run again. -- I have no doubt you will run again. With no news from the feds frustrations are growing. But later that day the break every one's been waiting for.
This transcript has been automatically generated and may not be 100% accurate.