When the smoke clears, what will be the real explanation why American intelligence and law enforcement authorities did not act on the Russian's tip about (Boston murderer) Tamerlan Tsarnaev?

Russians told the FBI about (the late)Tsarnaev more than once, including after the FBI interviewed him.

Now I will grant you, the Russians may have been worried about him for their own sake (he was after all a Chechnyan) and wanted the FBI to do their dirty work. And perhaps the FBI figured they didn’t feel like doing the Russian’s dirty work.

But I doubt it.

Was it incompetence? Was it standard human error? Was it timidity? Was it pride? What do you think?