Tehran has identified an array of parties as perpetrating or having a hand in the September 22 attack that killed 24 people in the city of Ahvaz.
Ahvaz is the capital of Khuzestan, a province bordering Iraq in southwestern Iran where ethnic Arabs form a majority.
There have been two claims of responsibility for the gun assault against a military parade, including one by the Daesh terror group.
Here is what we know about the accusations and claims.
- SEPARATIST MOVEMENTS -Al-Ahwaziya is an Arab separatist movement from Khuzestan province, but it comprises various groups.
On Saturday, London-based opposition channel Iran International TV broadcast a claim of responsibility by a group called the Ahvaz National Resistance.
Two other regional separatist movements -- the Ahwazi Democratic Popular Front and the Arab Struggle Movement for the Liberation of Ahvaz -- made statements denying any involvement.
- THE BLAME GAME -
President Hassan Rouhani on Sunday accused an unnamed Gulf country of giving financial, armed and political support to the instigators of the attack.
The same day, Iran's foreign ministry summoned the UAE's charge d'affaires over what it called "offensive remarks" by a political adviser. Abu Dhabi denied any link to the attack.
Iran has strained relations with the UAE, while diplomatic ties with Bahrain and regional rival Saudi Arabia have been broken since 2016.
On Saturday, Iran summoned diplomats from Denmark, the Netherlands and Britain, accusing these countries of hosting "terrorist group members" responsible for the attack.
In a statement on Sunday, the Revolutionary Guard pointed the finger of blame at a "Western-Hebrew-Arabic satanic triangle".
Foreign Minister Mohammad Javad Zarif said the attack was carried out by "terrorists recruited, trained, armed & paid by a foreign regime".
"Iran holds regional terror sponsors and their US masters accountable for such attacks," he wrote on Twitter.
Iran's supreme leader Ali Khamenei described the attack as the continuation of a "conspiracy by regional governments in the pay of the US".
- DAESH TERROR GROUP -
Daesh claimed its first attack in Iran on June 7, 2017, when gunmen and suicide bombers hit the parliament in Tehran and the shrine of revolutionary leader Ayatollah Ruhollah Khomeini, killing 17 people and wounding dozens.
On Wednesday, Daesh threatened to carry out new attacks in the Islamic republic.
Iran is "flimsier than a spider's web, and with God's help, what comes will be worse and more bitter", the group said in a statement on the Telegram messaging app.
In a video released in March 2017, the Daesh militants threatened to retaliate against Iran for its military and logistical support to the Syrian and Iraqi governments, as Damascus and Baghdad battled Daesh insurgencies.
The terror group said they wanted to conquer Iran to "return it to the Sunni Muslim nation" and to provoke a Shiite bloodbath.
