At least 15 people were killed in an armed attack on a major Shiite pilgrimage site in the southern Iranian city of Shiraz on Wednesday.
"A terrorist" attacked the Shah Cheragh mausoleum, said the head of the local judicial authorities, Kasem Mousavi, on Iranian state television.
According to eyewitnesses, there was a large contingent of police and security forces around the shrine.
The jihadist militia Islamic State (IS) claimed responsibility for the attack on its own propaganda channel.
An IS member shot at visitors to the mausoleum, killing "at least 20 Shiites and injuring dozens more," it said.
Iranian state television said a man with "connections to 'Takfiri' groups" had been arrested.
The term "takfiri" is used by the Iranian authorities to refer to Sunni jihadists.
According to the governor responsible for Shiraz, Mohammad-Hadi Imanieh, the attacker "shot blindly at the believers".
According to further information from the Iranian media, security forces injured the attacker, who is currently being operated on in a hospital.
Iranian President Ebrahim Raisi in a statement accused "enemies of Iran" of trying to use force to "divide the united nation".
Raisi promised a firm response from the security forces.
Shah Cheragh mausoleum is the most popular pilgrimage site in southern Iran.
In Iran, Shiite Muslims make up the majority of the population.
In Shah Cheragh is the tomb of Ahmed, the brother of the eighth Shia Imam Reza.
In early April, a 21-year-old visitor from Uzbekistan stabbed two Shia believers and injured another in the courtyard of his tomb, the Imam Reza shrine in Mashhad, Iran's second largest city.
The assailant was later found guilty of the crime of “war against God” (moharebeh), according to judicial authorities, and hanged in Mashhad in June.