I used to think it was a mystery, but I'm convinced now that it's only because they don't want to rile the country up as a whole, at least not just yet. I'm sure they know everything there is to know, and are working behind the scenes though to stop the threats from continuing.
They just don't want to deal with an incensed populace just yet--those of us who seek out the truth between the lines so to speak aren't a large proportion of the US population.