(e.g. "Hey, you might want to keep an eye on those Tsarnaev brothers -- see attached description of the stuff they were doing while they were still here in Russia.")
With my most elaborate tinfoil hat on, I wonder if the FBI delberately did not stop the Tsarnaev brothers.
In what field does one get more resources because you failed? Perhaps some people think that the price of the FBI having better tools (and the rest of us having less privacy) is the death of a small number of people at a high profile event?
Perhaps someone thinks that the price of stopping the next 9/11 plot is to let a smaller plot go ahead.