Since [9/11], we have learned that the radical ideology behind 9/11 is not quite as alien as we thought. Some portion of the American Muslim community - presumably small, but we don't know how small - is drawn to it.
Moreover, what looks like a considerable portion of those who hold themselves out (and are held out by the MSM) as leaders of American Muslims refuse to disassociate themselves from terrorist groups. They don't countenance al Qaeda, though they do blame America for that outfit's terrorist acts. But they won't repudiate other bloody terrorists, notably Hamas.
Thus, while only the most highly informed Americans probably could have imagined terrorist plotting or even pro-terrorist rhetoric in an American place of worship back in 2001, many can imagine it now, and with reason.