A group linked to al-Qaida is claiming responsibility for a suicide attack that killed 28 outside a Baghdad police training academy Sunday, according to a statement posted on an Islamist Web site.

The Islamic State of Iraq — an umbrella organization for several Sunni insurgent groups including al-Qaida in Iraq — said in the Internet statement that a suicide bomber wearing an explosives vest carried out the attack. The message’s authenticity could not be verified.

"A knight of the Islamic State of Iraq from the martyrs battalion ... on March 8, 2009, infiltrated with his explosives vest a big crowd of apostate police recruits near the gate of the police academy in Baghdad," it said, according to reports.

The attack, the worst in months, came after a long period of calm and was followed by one that killed 33 people Tuesday, prompting worries that violence was again on the upswing.

The Sunni insurgency played a major role in tearing apart Iraq soon after the invasion. It was composed of a mix of tribal leaders, members of the previous regime and jihadists. Those fractures led many Sunni groups to seek a truce with the Americans and turn on their former allies.

U.S. leaders eventually agreed, and Sunnis have become increasingly involved in the political process.

This week’s bombings make some worry about those gains, though. Some fear that the Iraqi military isn’t ready to protect the population at a time when the remaining insurgent groups are stepping up attacks to stop reconciliation between Sunnis and Iraq’s Shiite-dominated government. U.S. leaders say that violence still remains at historic lows and that insurgent groups are striking out of desperation at Iraq’s success.

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