A group that monitors militant websites says the al-Qaida affiliate in Iraq has claimed responsibility for a wave of car bombings that killed 72 people and wounded nearly 260 earlier in the week during a Shi'ite Muslim religious commemoration.

The U.S.-based SITE Intelligence Group said Saturday the Islamic State of Iraq posted a statement on militant forums saying it was behind the bombings. SITE said the militant group praised the attacks as the “blessed Wednesday invasion” that dealt a blow to the Iraqi government.

Most of the bombs exploded in Baghdad and the southern Iraqi cities of Hilla, Karbala and Haswa – predominantly Shi'ite areas that have been targeted before by Sunni Islamist insurgents. The bombings marked one of the bloodiest days since U.S. troops withdrew from the country.

The attacks came as pilgrims were headed to shrines to mark the eighth century death of revered Shi'ite religious figure Imam Mousssa al-Kadhim, a great-grandson of the Prophet Muhammad. The commemorations culminate on Saturday.