"Police have arrested 12 members of the group responsible for the attack against the church," an interior ministry official, speaking on condition of anonymity, said Saturday.
Huthaifa al-Batawi was among those arrested; he is the Baghdad chief of the Islamic State of Iraq, the al-Qaeda front group which carried out the attack.
There are no specifics about where the people are being held, but they were detained in raids carried out in Mansur, an upscale west Baghdad neighbourhood, and on Palestine street in the capital's east.
In all, 44 worshippers, two priests and seven security personnel were killed during the attack on the church and the shootout after it was stormed by troops. About 60 people were wounded, and the five people who carried out the attack were also killed.