Classified as extremists are hate preachers (the Germans are allowed to go into mosques and have found many imams preaching death to Germans, etc. I believe Americans still can't do that) or persons who are a threat to society (i.e. having shown that they have exhibited the will and expertise to undertake an attack at any time).
This information is from Bavaria,
Z: We in America, of course, are hand-tied by those who just can't imagine any imam would preach attack-inspiring or otherwise-threatening talk against us here in this country. Can we, too, err on our side by possibly arresting and expediting back home an islamist or two in order to protect millions of innocent Americans or is it just too unkind and would that perhaps threaten the rights of terrorists in our country? What do you think?