The attack on the al-Askariya shrine marks the first time that Iraqi sectarian violence has targeted one of the country's central religious symbols.

The Shia Muslim shrine has existed in the middle of the ancient city of Samarra, one of the largest archaeological sites in the world, since 944, when it was built to house the tombs of two ninth century imams, direct descendants of the Prophet Muhammad.

Ali al-Hadi, the tenth imam who died in 868 and his son Hassan al-Askari who died in 874, were buried at the end of the turbulent period during which Samarra was built as the new capital of the Abbasid empire, briefly taking over from Baghdad, then the largest city in the world.

But the continued and intense religious importance of the site is connected to the 12th and final imam, the so-called "Hidden Imam" who Shias believe went into hiding in 878 under the al-Askariya shrine to prepare for his eventual return among men.

According to Shia tradition, the Mahdi will reappear one day to punish the sinful and "separate truth from falsehood". For many years, a saddled horse and soldiers would be brought to the shrine in Samarra every day to be ready for his return, a ritual that was repeated in Hilla, about 100 miles to the south, where it was also thought that Mahdi might reappear.

"It's one of the foremost important shrines in Iraq," said Alastair Northedge, a Professor of Islamic Art and Architecture at the Sorbonne in Paris who has just completed an archaeological survey of Samarra.

"Najaf and Karbala are the two most important shrines in Iraq but only slightly subsidiary to them are the sites in Samarra and Baghdad.

"The shrine is central for the Shia. This is not just a major cathedral, this is more than that, this is one of the holiest shrines."

According to Professor Northedge, the shrine was extensively rebuilt as Samarra withered over the centuries and power was restored to Baghdad. Modern-day Samarra, a tough, Sunni-dominated town in the middle of the Sunni Triangle north of Baghdad, fills just a fraction of the enormous ancient city built along the banks of the Tigris.

The latest remodelling of the shrine took place in the late 19th century, with the dome that was destroyed today added in 1905. Covered in 72,000 gold pieces and surrounded by walls of light blue tiles, the shrine attracts thousands of Shia pilgrims from across the world.

Despite being an active base of Sunni insurgents since the US-led invasion of Iraq in March 2003, the al-Askariya shrine had survived unharmed and largely unthreatened until today.

It managed to escape any damage when Samarra was retaken in the first major US and Iraqi combined offensive in October 2004, which was aimed at sweeping out the Sunni factions that had taken over the town. The shrine has also remained intact while other archaeological sites have suffered under US efforts to control the insurgency.

The 101st Airborne Division, which took over Samarra shortly before Christmas, has continued the policy of using bulldozers to create a mud wall around the town to make it harder for insurgents to move in and out.

Professor Northedge, who last met Samarra's director of antiquities at a conference in Paris in September, believes the attack to be the work of al-Qaeda related militants from outside the town.

In September, Sunni rebels in Samarra joined an unprecedented condemnation of Abu Musab al-Zarqawi's al-Qaeda in Iraq after the execution of a leading cleric in nearby Ramadi.

"It is really quite surprising that something like that has happened in Samarra," he says. "The people there have a a very, very powerful sense of community identity, they know how to act in their best interests."

"If you look at the resistance situation in Samarra, there are two general sorts: there are local fighters and there are al-Qaeda fighters and foreign jihadis," said Professor Northedge. "I'm absolutely certain that this is not the local people from Samarra, they would not have blown it up."

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