The Iranian authorities have accused a shadowy group called Jundullah - the Soldiers of God - of carrying out a series of attacks, including a suicide bombing on 18 October 2009 which killed six commanders of the Iranian Revolutionary Guards. But what is Jundullah, what does it want - and who is behind it?
It was founded in 2002 to defend the Baluchi minority in the poor, remote and lawless region of south-east Iran.
Its leader, Abdolmalek Rigi, denies the group has either foreign links or a separatist agenda.
In an interview in October 2008, he said the group - also known as the People's Resistance Movement - was not interested in trying to break away from Iran.
It simply wanted the state to respect the human rights, culture and faith of the Baluchis.
Nation without a state
The Baluchis in Iran - and their brethren across the border in Pakistan - see themselves, rather like the Kurds, as a nation without a state.
But in predominantly Shia Iran, the issue is complicated by the fact that they are Sunni Muslim.
This has led them to claim sectarian persecution - and the Iranian authorities to accuse them of being in league with foreigners.
The list of powers alleged to be supporting them is a long one.
It includes the United States, Britain, Pakistan and Saudi Arabia - and militant groups such as the Taliban and al-Qaeda.
During the Bush administration, there were allegations - for example, by the investigative journalist Seymour Hersh - that the CIA was supporting Iran's Baluchi, Kurdish and Arab minorities to undermine the Islamic Republic.
If there was such a policy, it is not clear if the Obama administration has scrapped it.
It was quick to condemn Sunday's attack as an "act of terrorism".
An al-Qaeda link?
The Iranians also suspect covert support for Jundullah is coming from Pakistan's powerful intelligence service, the ISI.
But the two states have in the past co-operated in suppressing Baluchi nationalism - and also have important economic ties - so it seems unlikely Pakistan would want to antagonise its powerful neighbour.
As for the Saudis, given their resentment of Iran's newly enhanced role in the Middle East, it is not impossible some quiet assistance is going to Sunni groups like Jundullah.
Less plausible is a link to al-Qaeda.
Although Jundullah has recently adopted the jihadi tactic of suicide bombings, it seems more accurate to characterise it is a nationalist group with local grievances than part of Bin Laden's global jihad.