At least 15 people were killed and ten were injured in the attack on the Shah Cherag Mosque in Shiraz, the Iranian state news agency IRNA reported.
Witnesses point out that the attackers from a car shot at the pilgrims and staff at the entrance to the shrine. The police arrested two men, while the third is still being sought, reports Reuters.
IRNA said the attackers were acting as "Takfiri terrorists" - a reference to Sunni extremists such as Islamic State militants.
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