The difficulty in settling on either domestic nutjob or Middle Eastern terrorism is that it might be neither. Yet the media and LE seem to be in either/or mode. IMHO, everything we've seen is wholly compatible with an American citizen who's also a radical islamist. It also explains why the shooter's calls went unrecognized so consistently.
By accounts, the shooter was rebuffed by the FBI and/or Montgomery County at least six times, presumably by six separate operators. They can't all be inept. But they can all be constrained to checklists and fill-in-the-blank forms.
In a case of this magnitude and volume, call screeners, by necessity, employ some sort of formulaic "scorecard" to triage the thousands of calls pouring into the tip lines. Yet the real shooter couldn't get past them. He hit none of the filters or triggers.
Maybe there simply was no box to check for "radical Islamist sentiment spoken in perfect English with demand for money."