By Jim Muir BBC, Baghdad

In a country where things often turn out to be different from the way they at first seem, the retrieval of around 60 corpses from the Tigris River near al-Suwayra, south of Baghdad, is one of the murkiest and most complex stories to hit the headlines in recent months.

At first, it seemed straightforward enough. A few days earlier, there were persistent reports of a "hostage drama" in the mixed Sunni-Shia town of Madain, about 30km upstream.

The discovery has heightened fears of sectarian trouble

Sunni militants were alleged to have captured as many as 150 Shia inhabitants and to be holding them hostage, demanding that the entire Shia population move out of the area.

Spurred by angry pressure from Shia political factions, Iraqi government security forces backed by US troops arrived in strength on Monday. They encountered no resistance and found no trace of hostages or hostage-takers.

Two days later, the Arabic-language TV station al-Arabiya broadcast an on-the-spot report from al-Suwayra, quoting local police as saying that 58 bodies, all of them murdered, had been pulled from the river.

Not that simple

The connection seemed obvious and Iraq's newly-elected transitional President, Jalal Talabani, appeared to confirm it when he said emphatically that the bodies were those of hostages who had been murdered.

But closer investigation made it clear that it was not quite as simple as it seemed.

Senior police officials at the regional headquarters for the area gave a detailed breakdown of when the bodies had been found.

They said they had started to appear in the al-Suwayra stretch of the Tigris nearly two months earlier, on 27 February. On the first three days, 27 bodies were retrieved, while during and after the supposed hostage crisis only six corpses were pulled from the river.

But in the 26 days between 26 March and 20 April, there was a steady flow of cadavers. A total of 33 were retrieved during that period, an average of just over one a day.

The police statistics said that of 60 bodies 56 were men, two women and two children. Fifty-three had died of gunshot wounds, five had had their throats cut and two were beheaded.

Only seven of the corpses were identified by relatives. The remainder were photographed, numbered and buried in unmarked graves.

Angry denial

So the identity of the bulk of the victims was not clear. Were they all Shia Muslims, abducted and murdered by Sunni militants, as the immediate context of the news reports of hostages appeared to suggest?

It could take a long time to establish the bodies' identities

The al-Qaeda faction in Iraq, headed by the Jordanian Abu Musab al-Zarqawi, angrily denied the reports of mass kidnappings, accusing the Shia factions of fabricating the reports to justify a major security operation to seize control of the area.

Senior security officials in the Iraqi government said that there had been communal tensions in the area for weeks, based on both sectarian and tribal rivalries.

But radical Sunni militants linked to the insurgency had also been active in the area, and so, according to Iraqi officials, had armed elements from the Badr Brigade, a Shia militia linked to one of the clerical-led factions which won the biggest share of votes in the January elections.

It may be a long time before the identity of all the corpses is established, if it ever is, and the remains of other victims may not even have been found.

Relatives of people who have gone missing in the area in recent weeks have been flocking to the al-Suwayra police station to examine the photographs of the buried corpses.

Restraining hand

Some officials said the violence had not all been one-sided, and that tit-for-tat revenge killings may have taken place.

The campaign has so far failed to trigger the kind of widespread sectarian dissension that is apparently one of its goals

However, the Sunni insurgents are the major losers from the takeover in the area by government forces, as they will have lost a base from which they appear to have operated relatively freely.

Although the full story is not known, the kind of tensions that have clearly erupted in the Madain area - which is roughly equally divided between Sunnis and Shia - have heightened fears of sectarian trouble in a patchwork country held together for the past three decades by the iron grip of a ruthless dictator.

But despite the extraordinary provocations launched by Sunni insurgents against hallowed Shia targets, major communal clashes have somehow been avoided.

The major credit for that must go to the Shia clergy, and particularly Grand Ayatollah Ali al-Sistani, for urging their followers not to seek revenge or be dragged into sectarian confrontation.

Stress and grudges

The most visible and vocal strand of the insurgency is the "jihadi" element of radical fundamentalist Islamists, many of them non-Iraqi Arabs coming from outside, chief among them Zarqawi.

His group, known as the al-Qaeda Organisation in the Land of the Two Rivers, has claimed responsibility for many of the most gory and provocative attacks on the Shia community and its symbols. The group's propaganda has openly promoted hatred of the Shia as apostates.

The campaign has so far failed to trigger the kind of widespread sectarian dissension that is apparently one of its goals.