 This has led to growing concerns about al-Maliki's unwillingness to eliminate Shia militias.The Mahdi Army, a group linked to Iraqi Shi'ite cleric Muqtada al-Sadr, is held responsible for "execution-style killings" of 11 Iraqi troops in August 2006.Some U.S. officials posit that the militias are a more serious threat to Iraq's stability than the Sunni insurgency.