This picture of Peter Moore was taken on Thursday 31 December
Five Britons kidnapped in Iraq -including Peter Moore who was freed on Wednesday after two-and-a-half years - were held in Iran, it has been claimed.
Mr Moore and his four bodyguards were taken to a camp in Iran within a day of being seized, says the Guardian paper.
Three of the bodyguards were killed; the fourth is also thought to be dead.
However one Iraqi MP whom the Guardian said had been a key negotiator in the crisis has strongly denied Iran had any involvement in the hostage-taking.
The UK Foreign Office said it had seen the "speculation" about Iran's role in the kidnappings.
A spokesman added: "Iran of course has an influence in Iraq, but we have no evidence to substantiate claims of direct involvement in this case."
BBC security correspondent Frank Gardner
The findings in the Guardian's year-long investigation into alleged Iranian involvement in kidnapping Britons in Iraq are being disputed by both British and Iraqi government officials.
A senior Foreign Office official said that while it was "not impossible" that the British hostages had, at some stage, been taken across the border into Iran, that did not mean the Iranian authorities themselves were behind the kidnapping.
The British government view remains that there is no firm evidence to suggest Iranian government involvement.
But the Guardian is sticking by its story which includes interviews with a number of people who say Iran's Revolutionary Guards were behind the kidnapping.
The Foreign Office agrees there are close links between the kidnappers and Iran's Revolutionary Guards who have provided them and other Iraqi militias with training, funding and weapons.
The BBC's Jim Muir, in Baghdad, said there were many versions of what had happened.
"It's an extremely murky business, we may never know the truth," he said.
BBC security correspondent Frank Gardner said when he asked US General David Petraeus - the US's top military commander in the region - for his view this month, he said he thought "it was 90% certain" the Britons had been held in Iran.
But our correspondent added that this was "not same as saying that this was directed from the top by the Iranian leadership".
"My contact works for al-Quds. He took part in the planning of the kidnap and he watched the kidnapping as it was taking place. He told me that they spent two days at the Qasser Shiereen camp. They then took them deep inside Iran."
The newspaper also said a serving Iraqi government minister with close links to Iran had told its reporter it was an Iranian Revolutionary Guard operation.
"You don't think for a moment that those militia groups from Sadr City could have carried out a high-level kidnapping like this one," he is quoted as saying.
But Sami al-Askari - an MP and a senior adviser to Iraqi Prime Minister Nouri Maliki - has denied the Guardian's allegation that Iran masterminded the kidnap.
The newspaper also alleged that as the chief negotiator he flew to Iran several times to talk with the hostage-takers, which he has also denied to the BBC.
Mr Askari said he had been involved in talks with the kidnap group inside Iraq, which were continuing over the whereabouts of the final hostage Alan McMenemy, who is thought to be dead.
THE GUARDIAN'S ACCOUNT
Kidnap takes place at Finance Ministry in Baghdad
Hostages taken to Sadr City
Guardian claims the hostages are later taken to various locations in Iran, including camps at Qasser Shiereen and Tehran Pars
Guardian journalist Maggie O'Kane - who was involved in the investigation - told the BBC: "We are absolutely confident of the work we have done on the story."
Frank Gardner said the paper's investigation did not provide conclusive proof the men were taken to Iran.
But he said the witnesses appeared to be credible and some had been re-interviewed by the BBC.
He said it had long been known the Shia group who had been holding the men had links to Iran's Islamic Revolutionary Guards Corps.
"But British officials have always been reluctant to point the finger at Iran," he said.
He said it was thought that the kidnappers had separated Peter Moore - an IT consultant from Lincoln - from his four bodyguards early on in the process.
It was "perfectly possible" that Mr Moore would not know where he or the others were held, he added.
Brown thanks Iraqis
On Wednesday, Foreign Secretary David Miliband said Mr Moore, who was captured in Baghdad in May 2007, was in good health and "absolutely delighted" to be free.
He spent the night at the British Embassy in Baghdad, where he is receiving medical attention and support as he prepares to return to the UK. He will also receive formal debriefing sessions.
Downing Street said Prime Minister Gordon Brown had spoken to his Iraq counterpart Nouri Maliki on Thursday to thank him for his efforts in the release of Mr Moore.
"They also discussed shared concerns for Alan McMenemy," said a spokesman.
Frank Gardner: "Relations between London and Tehran could hardly get any worse"
The bodies of Mr Moore's British guards Jason Swindlehurst, from Skelmersdale, Lancashire, and Jason Creswell, of Glasgow, were returned to the UK in June 2009, followed by that of Alec MacLachlan, of Llanelli, Carmarthenshire, in September.
Mr Miliband said British officials believed Mr McMenemy, from Glasgow, was probably also dead and called for the immediate release of his body.
Mr Moore had been working for US management consultancy Bearingpoint in Iraq, while the other men were security contractors employed to guard him.
The kidnappers were understood to belong to an obscure militia known as the Islamic Shia Resistance, which demanded the release of up to nine of their associates held in US military custody since early 2007.
Several had already been handed to the Iraqi government and some had since been freed under the reconciliation process.
Mr Miliband has insisted no "substantive concessions" were made to the hostage-takers by the UK.
Frank Gardner said a senior Whitehall official had confirmed Qais Al-Khazaali - the leader of the kidnap group - was released "very recently" by the US to the Iraqi authorities.
Mr Khazaali had been suspected of involvement in the kidnapping and eventual killing of five US soldiers, he added.
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