Iranian authorities have arrested three suspects over the deadly bombing of a military parade in 2010, the state news agency has reported.
The IRNA on Sunday said the arrests were made in the Kurd-majority city of Mahabad, where 12 spectators were killed and dozens injured in the September 2010 bombing.
The report did not identify those arrested but said they were affiliated to Koumaleh, a Kurdish armed group which has been fighting Iranian forces for decades.
Iran arrested several people shortly after the attack, variously reporting that they were working with "Zionists", the United States and Baathists loyal to the former leader of Iraq, Saddam Hussein.
Iranian forces also said in 2010 they had killed suspects in the bombing near the border with Iraq.
Mahabad was the capital of a short-lived Soviet-backed "Republic of Kurdistan" in 1946, which was crushed within a few months. It was also the centre of a Kurdish uprising shortly after the 1979 Islamic Revolution.
Iranian media have often reported clashes between the elite Revolutionary Guards and Kurdish fighters said to be members of the Party of Free Life of Kurdistan (PJAK), an offshoot of the Kurdistan Workers Party which began an armed campaign in 1984 for an ethnic homeland in southeast Turkey and northwest Iran.
Several armed groups hostile to the establishment are active in Iran, including Kurdish separatists in the northwest, Baluch fighters in the southeast and some Arabs in the southwest.
The Sunni Muslim armed group, Jundollah, which Iran says is linked to al-Qaeda, is the most active. It killed 28 people, including Revolutionary Guards, in a 2010 double suicide bombing in retaliation for the execution of its leader.