The Islamic State group claimed responsibility for Tuesday's bombing of a bus carrying members of Tunisia's elite presidential guard that killed at least 12 people.
In a statement posted on social media Wednesday, the Islamic State group said a militant it named as Abu Abdullah al-Tunisi carried out the attack, infiltrating the bus and killing "almost 20 apostates."
The bus was traveling in the center of Tunisia's capital, Tunis.
The Interior Ministry said Wednesday the attack was carried out using 10 kilograms of explosives hidden in a backpack or suicide vest.
A ministry statement said a 13th body found on the scene is believed to be the "terrorist who caused the explosion," adding that DNA tests are under way to identify the attacker.
Tunisia also announced it is closing its land border with Libya for 15 days to guard against further attacks. Authorities say they believe thousands of Tunisians have traveled to Libya, Iraq and Syria to fight alongside Islamic extremists.
The United States on Wednesday condemned Tuesday's attack against members of Tunisia's security forces "in the strongest terms."
Also Wednesday, Tunisian President Beji Caid Essebsi urged his citizens to respect a curfew and other precautions that were implemented following the attack.
Essebsi said at an emergency National Security Council meeting Wednesday the only thing that should be on the minds of Tunisians is that the country is in danger. He said all of the current precautions are part of the "broader fight against terrorism."
"I would also like to stress ... that the war against terrorism is not only the responsibility of the government, nor only the security forces, nor only the army," he said. "It is a national responsibility."
Late Tuesday, Essebsi declared a 30-day state of emergency in Tunisia and Tunis was under an overnight curfew. Speaking on national television, he urged international cooperation against extremists who have staged several deadly attacks in recent weeks.
United Nations Secretary-General Ban Ki-moon and the U.N. Security Council both condemned the attack and expressed support for Tunisia.
The explosion came 10 days after authorities increased the security level in the capital and deployed security forces in unusually high numbers.
Earlier this month, Tunisian authorities announced the dismantling of a cell it said had planned attacks on police stations and hotels in the seaside city of Sousse, about 150 kilometers southeast of Tunis.
Tunisia's tourism industry has been hit hard this year following extremist attacks.
Shootings at a luxury beach hotel in Sousse last June killed 38 people, mostly tourists, while in March an attack by Islamist extremists at Tunisia's famed Bardo museum near the capital killed 22 people.