Did the FBI Just Prevent an Attack in DC from a Homegrown ISIS Supporter?

A very, very big arrest in Cincinnati today, involving allegations that a man named Christopher Cornell (online alias Raheel Mahrus Ubaydah) had planned to travel to DC in order to carry out an attack (via assault rifle) at the Capitol. It appears Cornell was arrested today after he purchased two ArmaLite M-15s. How did the FBI know? Because Cornell had been in communication with a confidential informant, someone who (according to the affidavit sworn in support of the criminal complaint) "began cooperating with the FBI in order to obtain favorable treatment with respect to his criminal exposure on an unrelated case." (affidavit para. 8(d)) As the affidavit further explains, the informant came across Raheel Mahrus Ubaydah posting pro-ISIS material on Twitter, and alerted the FBI. Cornell and the informant proceeded to open communications with each other (it is not clear from the affidavit whether this was at the FBI's direction), and Cornell claimed that he had been in touch with someone overseas who had not specifically authorized him to carry out an attack in the United States...but that he wished to do it anyway since al-Awlaki and others had blessed such attacks. Cornell allegedly added that his plan was to "make our own group in alliance with the Islamic State here and plan operations ourselves." (affidavit para. 8(f)). If the allegations in the affidavit are true, this is a disturbing reminder that the problem of homegrown would-be jihadis is by no means limited to France. And perhaps more significantly, it also is a reminder that it is not necessary for ISIS, AQAP, or any other formal group to be the moving force in funding, planning, or directing someone to carry out acts of mass murder; the practical barriers to an attack like this or the attack on Charlie Hebdo are not so high that one must have the direct involvement of such groups. The tricky part is the inspiration to act, which as we have seen can indeed be generated in indirect, diffused ways. Perhaps that is what happened here, though you can bet that the defense will argue that Cornell was not in fact independently inspired to carry out an attack, but rather was led into it by the informant. That familiar tension--between the immense utility of informants and cooperating witnesses when it comes to smoking out such threats in advance, and the real dangers of entrapment--will be a central point of contention as the homegrown threat evolves.