Al-Qaeda's Iraqi affiliate on Saturday claimed responsibility for a siege at a provincial council headquarters in Saddam Hussein's hometown in which 58 people were killed.
Tuesday's attack in Tikrit, a former stronghold of al-Qaeda, was the deadliest in Iraq this year. The assailants, who wore security uniforms and set off bombs, stormed the building and grabbed hostages, local officials said.
Hostages who did not die as a result of explosions were executed by the gunmen, they said.
In a statement posted on a website often used by Islamist radicals, the Islamic State of Iraq (ISI), a local al Qaeda umbrella group, said the attack was a response to what it said were crimes committed against Sunni prisoners.
Five al-Qaeda militants carried out the attack using a car bomb, explosive belts and hand grenades, the statement said.
The sectarian conflict between minority Sunnis and majority Shi'ites unleashed by the 2003 US-led invasion has largely subsided but a stubborn Sunni Islamist insurgency opposed to Shi'ite dominance of Iraq persists and attacks continue.
Al Qaeda has been strategically weakened by the deaths of leaders, and both its numbers and the territory in which it can manoeuvre have shrunk since 2006-07, when Sunni tribal chiefs turned on it and joined forces with the U.S. military.
But they are still able to carry out attacks aimed at grabbing attention and rattling the population at a time when Iraqi forces take centre stage as U.S. troops prepare to withdraw by year-end.