President Obama has acknowledged that “the Second Amendment guarantees an individual right to bear arms.” It follows from that, I think, that “suspect[ing]” someone — even suspecting someone of terrorism — isn’t reason enough to deny him that individual right. (Of course, that’s especially so given that such suspicion can snare people who share the same name as people who are suspected of a crime, and other people who aren’t actually guilty of anything; that’s the consequence of operating based on suspicion rather than based on proof.)
For more on this, see this post (which also briefly touches on no-fly lists, the extremely limited procedures for getting people off the lists, and other related matters).