This is not at all an insignigicant question!
According to Richard Pipes, 5-10% of American-based Muslims could be in that category. That's a lot of people, not to mention a big sea for the fish to swim in.
But that figure does not count our indigeonous Farrahkahners, Islamic-born-again convicts, unknown Islamic border-crashers, organized street gang opportunity-seekers, university sympathizers, Liberals and other assorted hard-leftists, fascists and orphaned Communists.
All in all, a lot of potential killers are roaming around unfettered out there in The Homeland just waiting for the opportunity to go public!