Fifteen people have been shot dead and dozens wounded in an attack at the Shah Cheragh mausoleum in the Iranian city of Shiraz, state news agency Irna says.
Initial reports said three assailants had fired at pilgrims and staff at the entrance Shia Muslim shrine, and that two had been arrested by police.
But the local judiciary chief said “only one terrorist was involved”. State TV also reported one arrest.
The Islamic State militant group (IS) said it had carried out the attack.
State media also alleged that “takfiri terrorists” – a label Tehran uses for Sunni Muslim militants like IS – were responsible.
Irna reported that two children and a woman were among the dead and published a photograph of bodies wrapped in sheets laid out in a courtyard. Other photos showed broken glass and pools of blood.
President Ebrahim Raisi vowed a swift and firm response.
“This crime will definitely not go unanswered, and the security and law enforcement forces will teach a lesson to those who designed and carried out the attack,” Mr Raisi said.
The mausoleum includes the tombs of two sons of the seventh Shia Imam Musa al-Kadhim, who are also the brothers of the eighth Imam Ali al-Rida.
Stéphane Dujarric, a spokesperson for UN Secretary General Antonio Gutteres, said attacks targeting religious sites are “especially heinous”.
In April, two Shia clerics were killed in a knife attack at a revered Shia shrine in the north-eastern city of Mashhad.
The assailant, who officials said was an ethnic Uzbek man from Afghanistan with radical Sunni views, was hanged in June.
His motive was not made public, but the interior minister at the time described it as a “terrorist attack” and blamed “takfiris”.
IS has carried out serious attacks in Iran before. In 2017, the group was responsible for a pair of deadly twin bombings that targeted the parliament building and the tomb of the Islamic Republic’s founder Ayatollah Khomeini.