British soldiers start testifying on Monday at a public inquiry in London into allegations that they tortured and executed Iraqis in an episode which, if confirmed by the investigation, would go down as one of the worst atrocities of the Iraq war.
Some 200 British soldiers are scheduled over the coming months to appear before the Al-Sweady Inquiry, named after one of 20 Iraqi men who died in disputed circumstances at or after a battle near the town of Majar al-Kabir on May 14, 2004.
The inquiry conducted three years of detective work in Iraq and in British military archives before starting oral hearings in March. Sixty Iraqi witnesses testified from March to June.
The aim is to establish whether the 20 Iraqis died on the battlefield, as the British soldiers say, or whether they were captured alive and later executed at a British military camp, as relatives and members of the local community allege.
The soldiers deny separate allegations by five men detained after the battle that they were tortured.
Named after a British checkpoint, the battle of Danny Boy took place the day after the Imam Ali mosque in Najaf, Shi'ite Islam's holiest shrine, was damaged during fighting between U.S. troops and radical cleric Moqtada al-Sadr's Mehdi Army militia.
With anger against foreign troops running high because of the Najaf incident, fighting broke out between a British convoy and armed insurgents on a road near Majar al-Kabir.
Several dead Iraqis were recovered by local people on the battlefield, but another 20 bodies were handed over by the British the following day at Camp Abu Naji, a nearby army base.
The soldiers say the 20 died fighting and their bodies were taken to the camp to check whether any of them were among suspects who were wanted over the killing of six British military police in Majar al-Kabir in an incident in June 2003.
They say the alleged executions were false rumors fuelled by the grief of relatives, the anger over Najaf and the unusual circumstance of the removal of the bodies from the battlefield.
This was denied by 15 Iraqi witnesses who travelled to London to give evidence and a further 45 who were flown to Beirut to testify via video-link from the British embassy there.
The military evidence is scheduled to last until early next year and the report from the inquiry - which has so far cost 19 million pounds ($29.6 million) - is expected in late 2014.
The inquiry has no power to prosecute. However, depending on what it concludes, the Service Prosecuting Authority which is in charge of military justice could decide to launch prosecutions.
Al-Sweady is the second major British public inquiry into the conduct of troops during the 2003-2009 occupation of Iraq, and it is likely to be the last of its kind.
The first, into the death of Baha Mousa in British custody in 2003, found the 26-year-old Iraqi had died in "an appalling episode of gratuitous violence". It made 73 recommendations to prevent future abuses of which 72 have been implemented.
One British soldier pleaded guilty to inhumane treatment and was jailed for a year before the public inquiry. No one else has been prosecuted, a fact condemned by human rights groups.
The Baha Mousa inquiry cost 25 million pounds over three years while the final bill for Al-Sweady is likely to be higher.
There are as many as 160 alleged executions and 800 cases of alleged ill-treatment outstanding from Iraq, according to a High Court ruling in May, but they will be handled differently.
The details of how to deal with them are being debated in court in complex litigation involving Iraqi claimants and the Ministry of Defense. A ruling is expected in October.