Tamerlan Tsarnaev: Mystery Surrounds Boston Bombing Suspect

How did a champion boxer turn into the alleged mastermind behind a violent terrorist attack?
3:00 | 04/24/13

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Transcript for Tamerlan Tsarnaev: Mystery Surrounds Boston Bombing Suspect
While there are still 34 victims of the Boston Marathon bombing in the hospital this evening one. In critical condition meanwhile one of the men suspected of maiming them all. Rests in fair condition under heavy guard so much of the investigative energies are focused on understanding why these two immigrant Brothers. Could turn fascination that took them in. And tonight ABC's Dan Harris follows a complex and chilling trail. Left by Tamil lives are now I have the elder an apparent mastermind. For awhile of the man police consider the mastermind of the maritime bombings seemed like a normal young -- the kind who made videos of himself joking around in Russian and posted a minute to. -- -- Who liked to Wear fancy clothes who is considered a hot shot boxer. The mystery one in which we are gathering fresh clues tonight is how this man who came to this country for a better life. Where he got freedom safety education and even welfare checks. How this man allegedly turned on America so merciless. Temporal and sent -- didn't start out so menacing theory is as a baby on his mom's lap that's he's dead to the left. This is Cameron and his siblings the little boy in front that's his little brother Jeff Carr who would allegedly grow up to be his partner in terrorism. Everyone moved to America with his family from -- -- and dangerous part of Russia at age fourteen. He went to a good high school and showed a lot of promise as a boxer but things started to go wrong not long after he graduated from high school. This is a mug shot from one camera -- was arrested in 2009. For allegedly slapping his then girlfriend. At the time he told police he was unemployed. He later met and married another woman named Katherine her friends have reportedly said -- and was abusive and controlling with her two. It was at around this time that family members say -- -- who was never much keen on religion before. Started to take a deep interest in is -- -- content isn't terrible thing. But I know that my keys have nothing to do with it's I normally. I mother. His mother says she pushed him in this direction because she wanted him to stop smoking and drinking so much I'm. Get involved is anything. That somebody radicalize them but it's not my brother. But his uncle told us today that there was another major influence in -- ambulance life. A mysterious man known only as Ni -- -- radical Muslim convert described as -- heavyset man with their red -- under -- -- -- -- and reportedly gave up boxing. And his study of music in Russia today Terrell -- mother told us the idea that nation negatively influenced your son. These nonsense. But it's pretty clear that whoever was influencing him -- -- was becoming increasingly severe he started to attend this mosque -- Cambridge. We're out of -- occasions he actually got into public fights with the imam and he stood up and also in a challenge in actually called. -- our preacher had occurred during this time the Russian government asked the FBI to investigate ambulance worried he had links to extremists in that country. I thought that these -- -- -- Hollis the FBI now says it found no threat and shut down the investigation perhaps what is now the most investigated period of Cameron's life. Is this six month trip he made to Russia last year. He went to his ancestral homeland to reach the decades long Muslim resistance against -- central government. The FBI which is now questioning -- parents over in -- wants to know whether the young man was recruited are trained. By any local Islamist groups during his time there when he got back home while he continued to subscribe to a conservative angry brand of islam's. There was something not so traditional Caroline was a stay at home dead. Taking care of his three year old daughter while his wife worked eighty hour weeks as a home health eight. The couple even went on welfare intermittently most recently last year. But despite the fact that the American government was supporting him despite the fact that this country -- given in so many opportunities. It appears that heroines -- and I have started planning his attack several months ago. In February he went to this fireworks store in Seabrook New Hampshire is one of the biggest flood itself that we have -- -- -- now in a hospital bed -- his little brother Jeff Carr. Reportedly says they learn how to make a bomb right up the Internet. Later. Well got this gun fight -- officers. That is them firing from behind their car Jack Black Mercedes SUV. According to the man who took these pictures from his bedroom. The photographer says he could see Caroline walked toward the officers firing until he went down and his brother got away. Remarkably tonight ABC news has learned that police believe during this entire shoot -- the semi automatic pistol -- Terrell -- hand was the only gun the Brothers had. Later recovered at the scene of the shoot out in Watertown. Which means that all day Friday as -- city of Boston which shut down the man holding it hostage justifiably considered to be armed and dangerous. Was at least as far as police can tell right now actually an unarmed nineteen year old curled up and leading in the back of a boat despite. All the evidence the boy's mother -- pushed them toward religion in the first place. Believes. They are innocent. Tonight her younger son is facing a criminal charge of using a weapon of mass destruction which could carry the death penalty. While Tara -- lies in a -- in more nobody has stepped forward. To claim his body. For Nightline this Dan Harris in Boston.

This transcript has been automatically generated and may not be 100% accurate.

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