Archaeologist Khaled al-Asaad was killed in Palmyra, Syria, after he reportedly refused to answer ISIS's questions about where ancient artifacts were hidden.
The U.K.-based Syrian Observatory for Human Rights says it isn't clear whether the move is meant to secure the ancient treasures from government forces or to destroy them.
The city is on UNESCO's World Heritage list and has been badly affected by the four-year civil war. The latest threat to Palmyra is posed by fighting between government troops and ISIS.