Saturday’s attack was the deadliest in the country in nearly a decade.
No group immediately claimed responsibility for the assault in Ahvaz.
The report described the attackers as ‘Takifiri gunmen’.
The Foreign Office fears more could follow the fate of Nazanin Zaghari-Ratcliffe.
Richard Ratcliffe has called for a meeting with the Iranian foreign minister.
Richard Ratcliffe has called on the Iranian foreign minister to meet him to discuss the case.
The British-Iranian mother was released from the Tehran jail on Thursday and had been staying with family outside the capital.
The mother-of-one was detained in Iran on spying charges five years ago while on a visit to her parents with her daughter, Gabriella.
Pictures have been posted online with Mrs Zaghari-Ratcliffe and her daughter.
What are the sanctions and how will other countries besides Iran be affected?