Why wasn't Tamerlan Tsarnaev on the FBI's watch list when he returned after 6 months in Russia?

On Today's Fox News Sunday:

CHRIS WALLACE, host: What do you make of the fact, because of a Russian request, the FBI interrogated the older brother Tamerlan back in 2011 about his ties to radical Islam. They found out he was not a threat.... [W]hat do you make of the interrogation? And what about the fact that when he returned after six months in Russia, he apparently was not on an FBI watch list?

CONGRESSMAN PETER KING: ... [W]here the FBI is given information about someone as being potential terrorists, they look at them, and then they don't take action. And they go out and carry out murders after this. So, again, I'm wondering, again, is there something deficient here? What was wrong? Again, there was nothing they could find in 2011. He goes to Chechnya in 2012. He has statements up on his Web site. He's talking about radical imams. Why didn't the FBI go back and look at that?... [I]s far as getting information in advance and not seeming to take proper action, this is the fifth case I'm aware of where the FBI has failed to stop someone who ultimately became a terrorist murderer....

Later in the show, Wallace interviews Philip Mudd, a terrorist expert with experience in the CIA, the National Security Council, and the FBI.

WALLACE: Was there any kind of a breakdown here in our national security operation, and specifically with regard to the FBI? Are you troubled by the fact that they were alerted by the Russians to the older brother, they interviewed him, decided he was not a threat, he goes to Russia, he comes back, and they don't seem to have him on a watch list?

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