Iranian state media reported that four gunmen dressed in military uniforms opened fire during a September 22 parade in Ahvaz marking the start of the 1980-88 Iran-Iraq war. Many held pictures of a four year old boy killed in the attack, one of the worst against the most powerful military force of the Islamic Republic.
At least eight of those killed were Revolutionary Guards personnel.
Describing the attack on defenseless people as an act of cowardice, the Leader noted, "According to [available] reports, this cowardly act has been perpetrated by the same people who are saved by Americans whenever they face a hard situation in Iraq and Syria" and whose operations are being financed by Saudi Arabia and the United Arab Emirates.
Top Iranian leaders also blamed the United State's Gulf Arab allies for the bloodshed that struck a blow at the heart of its security establishment.
Islamic State's Amaq agency posted a video of three men in a vehicle who it claimed were on their way to carry out the attack. The shooting was the deadliest terror attack to strike the country in almost a decade.
In his address, IRGC deputy head Hossein Salami reiterated accusations against the United States and Israel and warned the two countries to expect a "crushing and devastating" response from Tehran.
The Islamic Revolutionary Guards Corp has been the sword and shield of Shi'ite clerical rule in Iran since its 1979 Islamic Revolution.
"Considering (the Guards') full knowledge about the centers of deployment of the criminal terrorists' leaders., they will face a deadly and unforgettable vengeance in the near future", the statement said.
While no group has taken responsibility for the attack, local officials have been quick to point the finger at US, Israel or Saudi Arabia.
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Mourners flooded the streets of Ahvaz, the capital of oil-rich Khuzestan province, on Monday, waving Iranian flags and holding photographs of the victims aloft.
The U.S. government strongly condemned the attack and expressed its sympathy, saying that "the United States condemns all acts of terrorism and the loss of any innocent lives".
Iran summoned diplomats from Britain, Denmark and the Netherlands early Sunday for allegedly harboring "members of the terrorist group" that launched the attack.
A number of Iranian officials, including Zarif and President Hassan Rouhani, are expected to travel to Trump's hometown of New York City next week for high-level meetings at the United Nations General Assembly.
Nikki Haley, the USA ambassador to the United Nations, rejected Iran's accusations.
It is, however, highly unlikely the Guards will strike any of its foes directly and risk setting off a regional conflict.
Iran has said it has arrested a large network of suspects over Saturday's attack.
Shi'ite Iran is at odds with Western-allied Sunni Muslim Saudi Arabia for predominance in the Middle East.
It answers to Supreme Leader Ayatollah Ali Khamenei.