What Did the Russians Know and When Did They Know It?

The mystery surrounding the Tsarnaev brothers' ties to Russia is deepening, even as President Obama called Prime Minister Putin to offer thanks for their "close cooperation" in catching the suspects.

We know that Tamerlan Tsarnaev went to Dagestan, Russia -- a region bordering Chechnya and a hot bed of extremism -- for six months in 2012. We also know that prior to that trip, the Russians requested the FBI look into Tamerlan's background to see if there were any ties to terrorists. The FBI investigated but found nothing.

What happened when Tamerlan returned from his Russian trip? The chairman of the House Intelligence Committee, Mike Rogers, said on Meet the Press that there were limits to Russia's "cooperation" in vetting the elder Tsarnaev:

"It's important to understand why the FBI interviewed him in the first place: They were concerned about his possible radicalization," Rep. Mike Rogers (R-Mich.) said on NBC’s “Meet the Press,” adding the FBI did a “ very thorough job” trying to run that into the ground. “And then [the FBI] asks [for] some more help from that intelligence service to try to get further clarification."

"Unfortunately, that intelligence service stopped cooperating," Rogers said. "What happens is that case gets closed down."

The New York Times reports that the Russians “had something on him and were concerned about him, and him traveling to their region.”

What did the Russians "have on him"? How much info did they share? Evidently not enough to keep the FBI interested in him:

But the F.B.I. never followed up on Tamerlan once he returned, a senior law enforcement official acknowledged on Saturday, adding that its investigation did not turn up anything and it did not have the legal authority to keep tabs on him. Investigators are now scrambling to review that trip, and learn about any extremists who might have influenced, trained or directed Tamerlan while he was there.

House Homeland Security Chairman Michael McCaul is also wondering why the Russians never got back to the FBI about Tamerlan after he returned from Dagestan in July of last year:

After Tsarnaev came back, he created a YouTube channel with links to a number of videos–two of which were tagged under a category labeled "Terrorists" and were deleted. It's not clear when or by whom.

“If he was on the radar and they let him go–he's on the Russians' radar–why wasn't a flag put on him, some sort of customs flag? I've done this before. You put a customs flag up on the individual coming in and out. And I’d like to know what intelligence of Russia has on him as well. I would suspect that they may have monitored him when he was in Russia,” McCaul told CNN's chief political correspondent Candy Crowley.

Analysis of his social media accounts and interviews with his family members suggest Tsarnaev became increasingly radical in the last three or four years. But so far, there is no evidence of active association with international jihadist groups.

Might the Russians have evidence of Tamerlan's associations with terror groups? And why the interest in the young man by Russian intelligence before his visit? Why were they concerned about radical associations of Tamerlan ostensibly before he was radicalized? Had the young man been in touch with people in Dagestan who were already on Russia's terror radar?