Tonight's ET: Hear Deceased Tsarnaev's Voice

Tonight's ET: Hear Deceased Tsarnaev's Voice

ET has exclusively secured the never-before-seen documentary footage in which the deceased alleged Boston bomber Tamerlan Tsarnaev can be heard speaking for the very first time.

The footage of Tsarnaev is from an unfinished 2009 documentary about the Lowell, Massachusetts Golden Gloves boxing competition which he competed in and won. ET’s Rob Marciano also spoke exclusively to Tsarneav's trainer, John Allan, who was featured in the documentary.

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According to Allan, Tsarnaev, who was also a 2010 New England Golden Gloves boxing champion, became frustrated when he was not allowed to continue competing on a national level because of a rule change that required boxers to have citizenship. Allan spoke candidly to Marciano about Tsarnaev's father, his brother and alleged accomplice Dzhokhar Tsarnaev, his relationship with his wife and Tsarnaev's increasing interest in his religion.

"I think it disappears," Allan said about Tsarnaev's psyche in the boxing ring. "And he just does what he needs to do. At that point when the lights are on, there's no ego involved you know? His personality has nothing to do with it ... he's a machine. Clean, clear technique -- knows what he needs to do and just does it."

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And does Allan believe that the roadblock in his boxing career set him on the wrong path?

"Absolutely," Allan said unequivocally.

While he appears to have been close to Tsarnaev, Allan said that he never suspected that he could be a terrorist -- citing his level-headed demeanor in the ring.

"He had people that weren't as good as him go too hard [and] hit him in the face," Allan explained. "He'd smile, shrug it off. He could've dropped him if he wanted to. He was really respectful. It's bizarre. Or at least he appeared to be very respectful is what I should say."

Allan was in Thailand when he heard the news of the bombings. He couldn't believe his eyes when he saw pictures of Tsarnaev on television.

"First words out of my mouth was like, 'Oh my God, if that guy didn't have such a big nose, that would be Tamerlan,'" said Allan.

In the weeks following the Boston Marathon tragedy, Allen said that there hasn't been one day that's gone by where he hasn't wondered if he could have prevented the bombing from happening.

"I would like to think that I could have, or even more importantly maybe, on a human level, changed his mind about things," said Allan.

ET will be airing the documentary footage and the revealing interview with Allan all week beginning on Monday.