Either a left-wing nut case or a foreign terrorist who couldn't resist bringing his jacket into the country with him.
From the looks of it, I would guess it's the latter. It's good that for once they released actual pictures.
Forgive me if I'm a little skeptical. I have no doubt that there are terrorists in this country. However, a uniform for terrorists? I doubt it. Planted conveniently where it can be found? Terrorists are a bit more secret than that.
Don't discount disinformation/psyops. AQ knows processing intelligence is one of our weak spots. They've even been shown to plan based on the length of the feedback loop. From what I understand, one of their primary goals is to keep our intelligence assimilation capabilities stretched to the point of uselessness. False signals are much easier to plant than real opsec is to maintain. The latter is easier with more of the former.