Iraqi Prime Minister Haidar al-Abadi has ordered an investigation into the killings of three Sunni clerics and the wounding of two others near the southern city of Basra.
Abadi called on January 2 for the a swift probe and for the perpetrators to be arrested.
Police said the January 1 attack was carried out by four gunmen who opened fire from a car into the clerics' vehicle.
No one claimed responsibility for the attack.
A police colonel said one of the slain clerics had received death threats from members of the Islamic State (IS) extremist group.
Meanwhile, well-known Saudi Islamic State cleric Othman al-Nazeh al-Assiri was killed by an air strike in the northern Syrian town of Kobani on January 2.
The United States and its partner countries said they made 23 air strikes on IS positions in Syria on January 2.
Based on reporting by Reuters, AFP, and AP