BBC Analysis
By Martin Asser
Attacks on mosques or religious figures have the power to immediately bring to the surface the deep-rooted tensions between Sunni and Shia Muslims in post-Saddam Iraq.
Enraged members of each community often blame the other when their institutions come under attack, or they accuse Iraq's US-led occupiers of complicity or negligence.
After some attacks, people have also blamed Israel or, unlikely as it might seem, the US for the bloodshed.
Both Sunnis and Shias have been affected by violence with apparent sectarian overtones.
For Iraq's majority Shia population, the car bomb which killed leading cleric Muhammad Baqr al-Hakim and dozens of others in the holy city of Najaf in August 2003 was an event unsurpassed in its wickedness.
Other attacks have followed, culminating in the multiple bombings during the climax of the festival of Ashura, as hundreds of thousands of Shia Muslims flocked to shrines in Karbala and Baghdad.
Sunnis have had their own losses - on a much smaller scale - reflecting a nasty, slow-burning sectarian conflict that seldom makes its way into the news reports.
In just a few days in October, for example, the BBC learned of three separate fatal shootings against Sunnis coming out of mosques in Baghdad's western suburbs - all blamed by local people on Shia gunmen. None of them were widely reported.
The mosque is an obvious target for sectarian revenge or for anyone who wants to stir up religious tensions in Iraq.
Mosques are exclusively Sunni or Shia, therefore attackers run little risk of hurting people from their own denomination.
They are tempting targets for anyone wanting to stir up religious strife in the potential tinder-box that is post-Baathist Iraq.
Attacks on coalition forces, the civilian police, or international institutions, cause bewilderment or pride among Iraqis - depending on their individual viewpoints regarding the US presence.
But attacks on mosques have people out in the streets calling for revenge.
Suppression
Sunni-Shia tensions are hardly absent from any Muslim country where the two groups coexist in close proximity.
The tensions stem from the inherent rivalry between the two - a schism that saw the emergence of Shiism in opposition to the orthodox Sunni tradition some 1,400 years ago.
But in Iraq they are tied up with years of suppression of the Shia under Saddam Hussein's [Sunni] rule.
And now the two sides, along with other ethnic groups, are jockeying for power in a future Iraq whose constitutional make-up is up for grabs.
For those wanting to cause trouble for the Americans, there is the added advantage that mosque attacks cause great bitterness towards Iraq's occupiers.
Under international law, occupying powers are responsible for the safety of the population living under their control.
But as one commentator recently put it, the US army is far too busy protecting itself at the moment to worry about civilians in its care.
The lack of protection in such a sensitive area creates anger and resentment that fuels anti-American feeling.
It is with this backdrop that US forces say they found a 17-page document attributed to an alleged al-Qaeda figure urging attacks on Shia targets in order to radicalise "sleepy Sunnis" and drive them into al-Qaeda's ranks.
The author of the message - al-Qaeda-linked militant Abu Musab Zarqawi, according to the US - says the campaign must start before "zero hour" when the US hands over power to an Iraqi administration in June.
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