Gunmen assassinated a top aide of anti-American leader Muqtada al-Sadr, sharpening a Shiite power struggle that has already triggered fighting between the cleric's followers and the U.S.-backed Iraqi government. Riyadh al-Nouri, director of al-Sadr's office in Najaf, was gunned down Friday by an unknown number assailants near his home after returning from prayer services, police and Sadrist officials said. Al-Sadr blamed the Americans and their Iraqi allies for the killing but called for calm _ presumably to avoid a showdown at a time his Mahdi Army militia is under pressure by Iraqi and U.S.-led forces in Baghdad and southern Iraq. Prime Minister Nouri al-Maliki, also a Shiite, condemned "this savage crime" and ordered an investigation "to pursue and arrest the killers." But many of the 5,000 people who attended al-Nouri's funeral later Friday in Najaf chanted "al-Maliki is the enemy of God" as they shouted slogans against al-Sadr's Shiite political rivals.