Simplify the following text for a Grade 15 native English-speaking person.
Original: 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 .
Simplified: 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 .