Serbs , Jews , Roma and anti-fascists , including Communist and dissident Croat Byzantine Catholic priests , were interned in concentration camps , the largest of which was the Jasenovac complex , where many were killed by UstaÅ e militia . Serbs , Jews , Roma and Croatian anti-fascists , including Communist Croats and dissident Croat Byzantine Catholic priests , were interned in concentration camps , the largest of which was the Jasenovac complex , where many were killed by UstaÅ e militia .