Most crimes committed by US government officials in the course of carrying out their duties are documented in some way by the government. Those documents are subject to disclosure under FOIA; indeed that is the purpose of the FOIA.
That the FBI knew of a plot to murder American citizens and did nothing to warn the potential targets of such a plot; and furthermore since no one has, so far as we know been indicted, fired, reprimanded or otherwise held accountable for what is on the face of it a very serious crime... the obvious implication is that it was either some element of the government itself, or one of those monstrous public/private partnerships (like Xi/Blackwater/Triple Canopy/etc.) that is a "deniable violence provider" in the current global security context.
Now we can argue forever in the absence of further facts; but given the evidence we do have, it certainly suggests that we are looking at the very least, at a contingency plan to kill American Citizens on American Soil, for nothing more than exercising their rights under the 1st amendment.
The common understanding of Due Process as mentioned in the Bill of Rights specifically refers to the "due process of law"; and as such, the government deciding to kill American Citizens would seem to require a capital trial. In a court of law, before a judge with the usual 5th amendment rights to know the charges against one, and to hear and challenge the evidence on which the determination of their fate will be made; that is clearly not happening here.
We are clearly not living in the moral universe that we're taught about in civics class anymore. And while it's always been the case that some people "become cops to break heads", what we are seeing evidence of in the last few years is the emergence of an elite consensus that state violence of lethal intensity could be legitimately employed against unarmed civilians domestically as easily and as casually as it's deployed in Afghanistan or Iraq.