Gunmen killed 33 people, including 29 women, in a residential complex in a well-off section of Baghdad on Saturday.
Iraqi officials said the men raided a building in the neighborhood of Zayouna in the east of the capital, breaking into apartments and opening fire on the residents.
Initial investigations offered no evidence about the identity of the killers or the motive for the attack.
The attack is one of the worst in Baghdad since an insurgent offensive took over cities in northern Iraq, such as Mosul and Tikrit, and advanced on the capital. The killings raise fears that the civil war would descend into mass sectarian violence, such as that of 2006-2007 when death squads killed thousands of Shia and Sunni Muslims.
Until many Baghdad neighborhoods, Zayouna has a mix of Shia and Sunni residents, as well as Christians.
Video: Islamic State Parade in Kirkuk Province in Northern Iraq
Foreign Minister Urges PM Maliki to Apologize to Kurdish Leaders
Iraqi Foreign Minister Hoshyar Zebari has urged Prime Minister Nuri al-Maliki to apologize for claims that the leaders of Iraqi Kurdistan are maintaining “a headquarters for ISIS (the Islamic State), Baath, and Al Qa’eda and terrorist operations”.
Zebari told the BBC that unless Mr Maliki withdrew his remarks, Kurds would find it very difficult to work with him.
The Kurdish leadership suspended participation in Government, including that of Ministers in the Cabinet, in protest.
The office of Iraqi Kurdistan President Massoud Barzani went farther with a message to Maliki, “You must apologize to the Iraqi people and step down. You have destroyed the country and someone who has destroyed the country cannot save the country from crises.”
Tension has also risen in the past week over Kurdish occupation of oilfields near Kirkuk, with Iraqi ministers criticizing the advance of Kurdish peshmerga forces.
The Kurds responded that they are protecting areas, such as the oil-rich city of Kirkuk, because of the collapse of Iraqi forces.