By Paul Reynolds
BBC News Online World Affairs correspondent
In the aftermath of the attacks on the Iraqi Shias, attention is turning to whether this is the result of a deliberate attempt by elements linked to al-Qaeda to foment inter-community strife and civil war.
If there is a mastermind behind the attacks, then the main candidate would appear to be a man with links to Osama Bin Laden named Abu Musab al-Zarqawi. He is believed to be or to have been in Iraq and the Americans have offered $10m for his capture.
US Brigadier General Mark Kimmitt said that al-Zarqawi had to be "one of the chief suspects."
A copy of a letter said by US intelligence to be from him to his superiors in al-Qaeda abroad, was found on a computer disc in Iraq in January.
The tactic of attacking the Shias was laid out only too clearly.
The writer of the letter has a colourful, indeed ominous, turn of phrase. The letter says this of the Iraqi Shias:
"(They are) the insurmountable obstacle, the lurking snake, the crafty and malicious scorpion, the spying enemy and the penetrating venom.
"The unhurried observer and the inquiring onlooker will realize that Shi'ism is the looming danger and the true challenge. They are the enemy. Beware of them. Fight them."
Some of the hostility demonstrated in the letter to the Shias can be explained by the traditional antagonism between Shias and Sunnis. Osama Bin Laden and his followers are Sunnis.
Strategy for conflict
But the letter also goes beyond any doctrinal differences. It lays out a military strategy in which four enemies are identified - Americans,
Kurds, Iraqi security forces and the Shias.
Of the latter, the letter says: "These in our opinion are the key to change. I mean that targeting and hitting them in (their) religious, political and military depth will provoke them to show the Sunnis their rabies...and bare the teeth of the hidden rancour working in their breasts."
Then the writer expresses his hopes for provoking the Sunnis into retaliating.
"If we succeed in dragging them (the Shias) into the arena of sectarian war, it will become possible to awaken the inattentive Sunnis as they feel imminent danger and annihilating death at the hands of these Sabeans (a tribe regarded as non-believers).
"Despite their weakness and fragmentation, the Sunnis are the sharpest blades, the most determined and the most loyal when they meet those Batinis (heretics), who are a people of treachery and cowardice."
Dr Mustafa Alani, an expert on Iraq at the Royal United Services Institute in London said: "There must be a mastermind. We are not talking of a group of amateurs. The candidate for this is a well organised group. I believe that al-Qaeda or a group associated with it is a good candidate for this action."
Zarqawi is Jordanian by background and he is wanted in Jordan for questioning about the fatal shooting of a US diplomat in 2002. He fought in Afghanistan against the Russians (as did Osama Bin Laden) and was then wounded, and lost a leg, fighting for the Taliban in 2001.
The US Secretary of State Colin Powell claimed that he had been given refuge by Saddam Hussein and US officials linked him to an Islamic group in Iraq called Ansar al-Islam whose camps were attacked in the early stages of the invasion.
Not everyone believes either the letter or Zarqawi's role.
Dr Youssef Choueiri, reader in modern Middle East history at Exeter University, said that many parties had an interest in inciting trouble.
"The Zarqawi letter is discounted in many Arab circles," he said, "but if it is true and if there is a plan, it would fit into what has happened.
"But I myself believe that this is not to do with Osama bin Laden or loyalists to Saddam Hussein. There is a struggle going on in the Shia community between secularist and religious wings about the extent to which the constitution should be Islamic."
Dr Choueiri explained differences between Shias and Sunnis in political as well as doctrinal terms. Doctrinal differences go back centuries but modern political differences, he said, were manifested in attitudes towards the influence of religious authorities in a state.
"Sunnis say that religious leaders should be at the service of the state not in control of the state," he said.
The danger now is that the Shias will react as the writer of the "Zarqawi" latter wants them to.
So far, however, the Shia emphasis has been on political action, in particular in pressing a demand for early elections in which their majority numbers would count.
Their spiritual leader the Grand Ayatollah Sistani has managed to get the Coalition Authority to abandon its plan to delay elections until the end of next year and they are now likely to be held at the end of this year or early next year.
The attacks on the Shias came a day after agreement was reached on a temporary constitution to guide the interim government which is due to take over sovereignty in Iraq by the end of June.