Based upon Tamerlan Tsarnaev’s links to CIA operations in Caucasus , it appears that the Boston bombing was a CIA operation, and that Tsarnaev probably didn't know that he would be playing a part until shortly before it took place. This would explain why he didn't spill the beans in advance, and how he was able to get his hands on sophisticated "home brew" bombs - for all we know, he was assigned to buy the fireworks to give the appearance that he made the bombs, or the store where he supposedly bought them fabricated the claim that he bought them there for the same reason. As with Lee Harvey Oswald, he was evidently being given a certain profile and put through certain motions to give the impression that he was the bomber, but it's unlikely that the CIA would have left critical aspects of the operation up to him. They might have had back-up patsies in case Tamerlan screwed up.
Ominously, Obama yesterday implied that Tsarnaev was a "lone wolf," and that such operations are difficult to thwart. Here's a tip: monitor purchase of explosives, even if they're in the form of fireworks. But the government would of course know the importance of this, but it needs us to have the ability to buy explosives to explain its "lone wolf" terrorism.