Al Arabiya reports citing an official statement from the group, Censor.NET informs.
Tunisian authorities said that a suicide bomber carried out the attack on Tuesday, which killed at least 12 people and forced the government to impose a nationwide state of emergency.
Earlier on Wednesday, Tunisia's Interior Ministry said 10 kilograms of military explosives were used in the attack.
It said in a statement Wednesday that either a backpack or an explosive belt containing the explosives was used.
The 13th body is believed to be the "terrorist who caused the explosion," the statement said adding that the body can't be identified by fingerprints because no fingers were found. A DNA analysis of the body is underway.