Tunisian authorities said Wednesday they have discovered a 13th body in the bus attacked in central Tunis. The body is believed to be the "terrorist who caused the explosion," the Interior Ministry said.

The attack on the bus carrying Tunisia's presidential guards involved about 10 kilograms (22 pounds) of military explosives, the ministry added.

In a statement, it said either a backpack or a belt containing the explosives was used in Tuesday's attack, which rattled the country after a particularly violent year.

No one has claimed responsibility for the bombing, which the government called a terrorist attack.

Ministry spokesman Walid Louguini told The Associated Press on Wednesday that the 13th body couldn't be identified by fingerprints because no fingers were found. The ministry said a DNA analysis of the body is underway.

Troops fanned out around the capital and the president declared a month-long state of emergency after Tuesday's attack. Earlier this year, the country suffered two major attacks by Islamic extremists that targeted tourist sites.

The blast on a tree-lined avenue in the heart of Tunis is a new blow to a country that is seen as a democratic model for the region. It came days after authorities visibly increased security in the capital and deployed security forces in unusually high numbers.

The U.S. State Department denounced the attack and the U.N. Security Council pledged support for Tunisia's young democracy.

Iyad Madani, secretary general of the Organization of Islamic Cooperation — the world's largest bloc of Muslim-majority countries — strongly condemned the attack. In a statement Wednesday, Madani expressed his solidarity with Tunisia and said such acts of terrorism are seeking to alter the country's "moderation and tolerance-driven model of society."

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Aya Batrawy contributed to this report.