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And I'm Renee Montagne.
The world has heard some relatives of the Tsarnaev brothers.
INSKEEP: On this program, we heard an aunt of the young men defend the accused Boston Marathon bombers.
MONTAGNE: An uncle has very vocally denounced them.
INSKEEP: Now federal investigators have been interviewing the parents, both of whom now live in Russia.
NPR's Dina Temple-Raston is tracking the story. She's in our studios here in Washington. Hi, Dina.
DINA TEMPLE-RASTON, BYLINE: Hi, there.
MONTAGNE: So, Dina, what are investigators learning?
TEMPLE-RASTON: Well, U.S. officials have arrived in Russia, and they're working with security services there to create a timeline that accounts for six months when Tamerlan Tsarnaev actually visited Russia last year. The concern is that he attended a training camp while he was in Dagestan. And they're worried that that's where he learned how to build the bombs that eventually killed three people and injured scores of others last week.
INSKEEP: Dagestan, that's part of Russia...
TEMPLE-RASTON: It's part of Russia.
INSKEEP: ...in the Caucasus. OK, go ahead.
TEMPLE-RASTON: So far, they haven't been able to find any evidence that he attended some sort of training camp. What they're looking for are unaccounted absences, you know, days away from his father who lives there, when his father may not have known where he was. And so far, though that could change, they haven't discovered that kind of pattern.
And it's plausible. His father lives in Russia. He could have been there just visiting him.
INSKEEP: So one question is what the father knew, and when was he away from the father. Isn't it also true that Tamerlan was influenced by his mother, that she encouraged him to become more deeply religious?
TEMPLE-RASTON: Well, this is one the big questions about his apparent radicalization. When did he go from being a boxer and sort of a man about town, to being a very devout Muslim? And there's some indication his mother played a role. But she seems to have encouraged his return to Islam in a sort of a benign way. She was worried that he was getting in trouble with the law. And she thought it would be better for him to turn to Islam than to be a scofflaw.
Officials familiar with the details of her interview said that that is the reason why she asked him to embrace Islam. And she wanted it to be a positive thing. They also said - these officials - that it appears she had no idea that there was another layer to his beliefs, that he was increasingly turning to more radical forms of Islam.
And right now, they're looking for another man they believe might have encouraged his radicalization, someone who may have attended a local mosque in Boston with him. And it's unclear what role that man played, but the FBI is looking for him.
MONTAGNE: And, Dina, you mentioned that investigators are trying to determine if the brothers could have made these bombs on their own. Do investigators know anything more about the components of the bombs?
TEMPLE-RASTON: So, they're analyzing them now, but we have a couple of new details. The first is that the bombs appear to have been detonated by remote control. It's unclear if they were detonated by a cell phone or something else. You know, in the surveillance video that everyone's talking about that took place before the explosions, officials say they saw Dzhokhar with a cell phone to his ear, and he appears to have been talking.
The question is: Was he talking to his brother, or was he detonating the first bomb? Officials don't know yet.
And the second new development is that on the bombs is that officials say that at least some of the explosive material they found in one of the bombs is consistent with the black powder found in the kind of fireworks that Tamerlan is thought to have purchased ahead of the attack.
INSKEEP: OK. So a little bit is being learned about the bombs there. The other questions, I suppose, has to do with law enforcement officials themselves. We know that Russian authorities called - at least the older brother - to their attention a couple of years ago. How are law enforcement officials defending themselves?
TEMPLE-RASTON: Well, FBI officials actually interviewed Tamerlan and his parents several times back in 2011. The Russian government had asked for that interview, because they thought that Tamerlan was a possible terrorist threat to Russia - not to the U.S., but to Russia. And the FBI conducted three interviews and did this massive database search on him to see if he was - if he had any part in terrorist communications. And they came up completely empty. They say he hadn't done anything illegal.
Even so, he was added to a U.S. watch list known as the Terrorist Identities Datamart Environment, or TIDE list. Basically, it's a giant list of people. It's about 745,000 people as of last year. And it's basically a list of anybody who's been red-flagged for some reason. And it's the database that's drawn up to become the no-fly list.
INSKEEP: Is that the most they could have done, under the circumstance?
TEMPLE-RASTON: They might have done more, but they didn't have the illegal predicate to do it. So that's why he was put just on the TIDE list, as someone to basically watch.
MONTAGNE: And, Dina, were they actually watching him?
TEMPLE-RASTON: No. U.S. law enforcement knew about him. They'd even flagged him. But they never found enough derogatory information to put him on one of those higher-priority lists that would have allowed them to watch him all the time.
MONTAGNE: NPR's Dina Temple-Raston. Thanks very much.
TEMPLE-RASTON: You're welcome. Transcript provided by NPR, Copyright NPR.