Iran’s intelligence minister says a number of suspects have been arrested in connection with Saturday’s gun attack on a military parade in Ahvaz.
Mahmoud Alavi said a “large part of the network” behind the attack, which left 25 people dead, had been caught.
He was speaking at a funeral ceremony for the victims attended by thousands.
The deputy head of the Revolutionary Guards meanwhile warned the US and Israel to expect a “devastating” response, accusing them of involvement.
Iran’s president alleged on Sunday that the gunmen were armed by a Gulf state allied with the US. The US condemned the incident and denied any responsibility.
Ethnic Arab separatists and the Islamic State group (IS) have issued competing claims that they carried out the attack, which was the deadliest in Iran in almost a decade.
What happened on Saturday?
The parade in Ahvaz – the capital of the south-western border province of Khuzestan – marked the anniversary of the start of Iran’s 1980-88 war with Iraq.
Four gunmen reportedly wearing military uniforms opened fire on troops taking part in the event, bystanders, and a viewing stand for officials.
At least eight of those killed were Revolutionary Guards personnel. Women, children and a disabled war veteran were also among the victims, officials said.