Potential Warning Signs of Boston Marathon Bombing

New facts emerge about the two brothers at the center of Boston bombing.
4:34 | 04/21/13

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Transcript for Potential Warning Signs of Boston Marathon Bombing
One of those questions, were there any warning signs? In the case of the older brother, there may have been. This morning, we also know that the fbi interviewed him two years ago. Abc's senior justice correspondent pierre thomas has been following it all from washington. Reporter: Good morning. The older brother is the subject of intense focus today. My sources are telling me that tamerlan tsarnaev may be key to unraveling the key behind these attacks. In 2011, the russian government became concerned about tamerlan, 26, suspect number one, travel from the u.S. Back to russia to connect with radicals there. They asked the fbi to investigate. My son would never do this. It is a set up. He was controlled by fbi like for five, three, five years. They knew what my son was doing. Reporter: The fbi acknowledged that it interviewed tae taermlan at the request of the russians. After looking at his phone records, and websites he visited, he found no ties. Too many people in this country, just like him, that are touching extremist websites, espousing things that aren't particularly kind to americans. That's not against the law here. Reporter: Taermlan's father said that his sons couldn't have planned the attacks. They never knew that a dark side of the two brothers begun to emerge. Tamerlan was charged in boating a former girlfriend. Jahar began writing bizarre tweets. Authorities never saw this coming. Investigators are asking this morning, are there more cell members out there? That's the critical question, dan and bianna. Pierre, a high level of security that there may be co-plotters out there. Dan, think are trying to find out are there more people out there? So critical they talk to the younger brother who's still recovering from the injuries. They say they must find that out as quickly as possible. There's an international investigation under way to get to that subject. Time is of the essence, let's bring in george stephanopoulos, host of "this week." Are they forced now to consider launch a military response? I think that's getting way ahead of ours, based on everything we know, even if there were accomplices, those accomplices were tied to a cell overseas to be identified and targeted. Remember, they went back to russia, russia has been fighting these chechens separatists as well. I think military action is not being contemplated here in any way, shape or form. They are, as pierre said, was there a wider cell involved in this right now? No evidence yet. Immigration is going to be at the top of the list with this new reform bill being introduced, some talk that the bombings could complicate this?

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

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