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This week was not the first time the FBI had heard of Tamerlan Tsarnaev, the older Boston Marathon bombing suspect who died after a shootout Friday morning. The FBI now says that "a foreign government" - Russia, or more specifically the Russian FSB, successor to the KGB - asked the bureau to check him out in early 2011, warning that Tsarnaev was a "follower of radical Islam," and "a strong believer," and may have been planning to leave the U.S. to join an underground group.
Intelligence sources tell CNN it is "rare" for Russians to reach out like that, to ask the FBI to look into someone as they did with Tsarnaev.
The FBI interviewed Tsarnaev in 2011, but says it found no terrorism activity at the time. But it appears the bureau did not follow up after Tsarnaev spent six months in Chechnya in 2012. The question is, why?
"There's going to be a lot of questions, that's one of them obviously," said Rep. Michael Capuano, D-Massachusetts. "I'm sure the FBI is going to want to ask those questions of themselves, as well."
Th FBI "and the White House will look back and ask, 'What did we miss? How did we miss it? Did we not take the Russians seriously enough?'" said Juliette Kayyem, CNN national security analyst and Boston Globe columnist.
What is not known now is how many people the FBI interviewed, over what period of time, and whether there was any future surveillance. Also unknown is whether the Russian FSB gave the U.S. a list with many names on it, or just a few - whether it was a standard request, or something more unique that should have warranted more attention.
All of these questions will have to be answered, not only because the American public will ask them, but because the Obama administration will want to know what can be learned from this horrific event, said Kayyem.
"This could happen again, and we want to get better at ... stopping it from happening again," said Kayyem.
CNN reached out to the White House today to get more of an explanation about why the FBI did not follow up with an interview when Tsarnaev returned from Chechnya, and whether they felt the FBI was on the case enough. The White House deferred comment to the bureau, which did not answer questions Saturday.
"A bad thing happened, and what we don't know now is who did the investigation," said Kayyem, saying there are many ways to pursue such investigations - through the Joint Terrorism Task Force (JTTF), the FBI office in Boston, or the FBI office in Washington, D.C., for example.
The former DHS official adds that looking back, should be about preventing something awful from happening in the future.
"Sometimes when you put the pieces together at the moment it doesn't make as much sense, and that's what going back is about," said Kayyem. "It's not just about blame."
"The FBI clearly wants to get to the bottom of this as well, because they obviously don't want this to happen again," said Kayyem.