When it was learned that the FBI had checked up on Tamerlan Tsarnaev and failed to predict that he would become radicalized, some politicians implied that the agency might have “dropped the ball.”
Given that Tamerlan Tsarnaev did apparently turn out to a threat, it is tempting to infer that the FBI did drop the ball. Now that it is known that he was a threat, people are going back and reconstructing the evidence that he had become radicalized, such as his YouTube links and his outburst at a Mosque. However, this temptation should be resisted (unless evidence emerges to the contrary).
In regards to tracking people and predicting whether they will become a threat, the FBI faces two main philosophical challenges. The first is epistemic: that is, how do they know that a person will become a threat? This, as might be imagined, can be rather problematic. After all, as some commentators have noted, the FBI checks on many people every year and the vast majority of them do not turn out to be threats.
To use the obvious analogy, some people have mental health issues that might lead to serious violence, but the vast majority of such people never actually engage in such violence. When someone with such issues does engage in violence, people endeavor to backtrack and look for what was missed-and it always seems that the definitive evidence is never found. This might be because people have free will, because behavior is ultimately random, or because we lack the epistemic abilities to find the key evidence. Or something else entirely.
In the case of Tamerlan Tsarnaev, it might be found that there is no decisive evidence that would have revealed him to be on the (alleged) path to the bombing. That is, given the reasonably available evidence, perhaps the FBI lacked an adequate reason to expend its limited resources in tracking Tamerlan Tsarnaev in detail.
This possibility seems likely. As is often the case, the only definitive evidence that a person will engage in violence is when the person actually does so. Naturally, it would be rather useful to be able to definitively sort out the pre-criminals/terrorists before they act-but this is a rather difficult challenge given our capacity to know.
The second challenge is ethical and deals with such matters as the right to privacy and concerns about having a police state. While the state could keep closer checks on people who are even suspected of being potential wrong doers, there are obviously moral concerns with such an invasive state. The recent battle over expanding background checks for gun purchases showed the extent to which some people are concerned about matters of privacy and rights even in the context of public safety. After all, if there are significant concerns with expanding background checks for buying guns, then one can only imagine to concerns with having the FBI keeping close tabs on people on the basis of a foreign state making an inquiry about them and other such reasons.