The Syrian authorities are currently trying to attribute the blast Thursday on a bus carrying Iranian pilgrims near the Saida Zeinab mosque in Damascus to an exploding tire. However, eyewitnesses earlier reported a bomb explosion on the bus, killing a number of people and causing damage to buildings in the area. Syria's Interior Minister Mohammad Sammour ruled out a terrorist attack in a statement to state-run Syrian TV. He said the bus driver and two gas station workers were killed when a tire into which they were pumping air exploded. But a private Syrian television station, Ad-Dounial TV, said six people were killed in the blast, and Iranian state television also reported six killed, including two Iranian bus drivers. The tire story, on the face of it, looks like a somewhat ludicrous attempt by the Syrian authorities to explain away an alarming episode for the regime. If what took place in the Saida Zeinab quarter was in fact a bombing, rather than an exploding tire, then it may be assumed that the perpetrators were intending to deliver a series of calculated insults. First, and perhaps most importantly, such an act would constitute an attack on the Islamic Republic of Iran. The explosion took place as Saeed Jalili, Iran's chief nuclear negotiator, was visiting Damascus. The targeted bus contained Iranian citizens, at least one of whom is reported dead. This is the second apparent terror attack on an Iranian target in the last two months. The previous incident targeted Revolutionary Guard officers. Such attacks have the quality of making a regime, which prides itself on its ability to project force and defiance, look suddenly vulnerable. Iran prefers to sponsor, not suffer, the attacks of terror organizations. Second, such a bombing would be a slap in the face for the Assad regime. Syria has been emerging smartly from international isolation in recent years. Its practice of fomenting trouble for its neighbors - Israel, Iraq and Lebanon - and then offering to help solve the problems it is largely responsible for creating, has been paying dividends. But a security-state such as Ba'athist Syria holds power because of its ability to inspire fear and impose quiet at home. In the last two years a series of embarrassing events have served to tarnish the regime's image of chilly authority. The killing of Imad Mughniyeh in February, 2008 was the first of these. The subsequent death of General Mohammed Suleiman in August of the same year further reduced the Syrian Ba'athists' projections of invulnerability. In September, 2008, meanwhile, a car bombing on a security complex in a civilian neighborhood of Damascus took place. An attack on Iranian pilgrims in Saida Zeinab would be yet more embarrassing for Syria because it would indicate its inability to protect the citizens of its closest regional ally from sectarian attack on its soil. Third, a bombing of this kind would constitute an assault on Shi'ite Islam. It would bear the hallmarks of the sectarian attacks on Shi'ite targets which characterized the Sunni insurgency in Iraq. The Saida Zeinab shrine is a place of pilgrimage for Shi'ite Muslims everywhere. Saida Zeinab, a granddaughter of Mohammed, is venerated by Shi'ite Muslims as a heroine of the seminal battle of Karbala. Hence, a bomb near the site of the Saida Zeinab shrine would be an expression of Sunni contempt for the symbols held dear by Shi'ite Islam - and for the Shi'ite practice of venerating individuals associated with the early years of the faith. Now, assuming that a mysteriously potent puncture might not have caused the carnage at Saida Zeinab, what manner of organization could have been responsible? It is impossible to know for sure, of course, but the signs would suggest a Sunni jihadi grouping of some kind. Syria's relations with Sunni Islamists are complex. Damascus has offered support and safe passage to Sunni jihadis on their way to fight in Iraq. Yet the regime itself - non-Sunni, aligned with Shi'a Iran, and with a record of brutal suppression of the Muslim Brotherhood - is also a natural target of opposition for devout Sunni Islamists. The world of extreme Sunni Islamism is notoriously murky and riven, with many groups operated and/or supplied by governments for their own aims. It can only be a matter for speculation and theorizing (of which there will be much) as to who might have had an interest in striking a blow at Iran, its religion and its allies in the heart of a regional capital. But the latest events in Damascus offer further potent proof to Iran and Syria that support for terrorism is a two-way street.