Abu Kuther sits in the candlelight, casually flicking his left wrist and sending a long string of prayer beads arching into the air. "There is a famous Arabic saying," he says. "There are many ways to die but death is always the same."

Kuther, interviewed in a secret location in Baghdad for the Guardian, said he was one of the kidnappers of five Britons taken from a government ministry building in the Baghdad in May 2007.

He has told the Guardian that he was a member of militia group called the "Righteous League" – what he doesn't admit is that the league is a front for the Iranian Quds force created, trained and funded by Iran, and that the kidnapping of the five Britons was led by al-Quds force commandos.

The kidnapper spoke to the Guardian after being identified by a senior leader for the Righteous League and a member of the family of the man later exchanged as part of the deal which saw Peter Moore's freedom.

A year-long investigation by the Guardian can reveal that Iran's Islamic regime – and specifically the al-Quds force – was heavily involved in the kidnapping of the five men.

The Quds force runs overseas intelligence operations for the Iranian government.

The other Britons captured with Moore – all security guards – were Alec Maclachlan from Llanelli in Wales, Alan McMenemy from Dumbarton in Scotland, Jason Swindlehurst and Jason Creswell. The bodies of Swindlehurst and Creswell were identified after they were handed over in June, followed by Maclachlan's in September. McMenemy is also believed to be dead, though his body has so far not been returned.

Just one, Peter Moore, made it out of Iran alive.

Motives

And the Iranian regime's motives for the abduction can be confirmed: retaliation for the arrest of key Iranians in Iraq by the United States; anger at the direct threats against Iran by President George W Bush and the desire to prevent Moore, the computer expert among the five kidnap victims, from installing a sophisticated tracking system that would show how billions of pounds in international aid money from Iraqi institutions was diverted to Iran's militia groups in Iraq.

The narrative surrounding the hostage taking has been well-rehearsed.

On the morning of Wednesday, 29 May, 2007, Moore, a computer specialist working for BearingPoint contractors, arrived at the technology centre of the Ministry of Finance in central Baghdad. He was joined by another man, Peter Donkin, another specialist, and the four British bodyguards.

The job the team was working on was to install, and train staff on, a new software system which was designed to track billions in aid as well as the huge revenues generated by the Iraqi oil industry.

Iraq's ministries were notoriously corrupt and according to evidence given to the US Congress, as much as $18bn (£11bn) in aid and funds had gone missing in recent years.

The British team had been in the technology centre for only a matter of hours when between 80 and 100 armed men, apparently dressed in the uniforms of Iraq's ministry of the interior, arrived in a fleet of 4x4s and stormed the building.

One witness, Wihaib Allawi, a janitor at the Ministry, told the Guardian: "They kept shouting in English 'sit down, sit down' to everyone they saw."

Another witness, Haider Sa'adoon, an administrative assistant, claims the kidnappers "were talking in a language we couldn't understand".

Moore and the four bodyguards were bundled into separate cars and driven off at speed into the mayhem of Baghdad's traffic. Donkin, who was in the lavatory at the time the kidnappers arrived, escaped detection after the Iraqi staff tried to protect him by hiding him under floorboards. The kidnappers missed him.

So much is known. But what happened next can be revealed for the first time.

Unknown to the kidnappers, they were followed by undercover agents working for Iraq's intelligence service who, by chance, had been outside the Ministry of Finance when the men were taken.

A former senior Iraqi intelligence commander has told the Guardian that the agents, who worked for him directly, followed the convoy of cars into Sadr City.

It is here in the dusty, airless suburb of east Baghdad that the men spent their first night in captivity, in the al-Zahra mosque. That night British intelligence and the employers of the bodyguards, GardaWorld, searched Sadr City looking for them, desperately trying to confirm reports that they were hidden in a disused petrol station. GardaWorld admits to allowing the kidnap and ransom insurance of the four security guards to lapse due to a "clerical error" before the abductions.

The intelligence officers passed the news that the men were inside the mosque up the command chain at the Iraqi Ministry of Defence. It was still early in the afternoon, but it was to no avail as no action was taken. The next day – nearly 24 hours after the kidnapping – the prisoners were on the move again.

The Guardian's intelligence source said: "They left the mosque, the second day at 8am. Our two guys followed the cars ... they followed the cars at a distance and the cars went towards al-Obaidi, towards the brick factory. It is an area where several brick factories are and the road ends with a dusty route that leads towards the Iraqi-Iranian border ... it is the shortest way between Baghdad and the borders. It is a desert area full of sand hills, no one lives there."

He continued: "Past the brick factories, there were cars waiting for them. While they were transferring the hostages to the other cars, our two guys were hanging around, pretending that they were buying bricks for their houses. One of our guys was negotiating and the other was watching the cars.

"They saw when they moved the British hostages from one car to another – each one into a separate car. They saw that they were hooded and handcuffed. The cars went in the direction of the Iranian border – because the road is unpaved and dusty, it can take up to an hour and a half to reach the border. But the road they took only leads only one way – to Iran."

There was a window of some 17 hours to rescue the five British men but Iraqi intelligence officials were told not to tell anyone what they had seen.

"They immediately told their boss, who called his boss at the Defence Ministry," according to the Iraqi intelligence source. "The following day a message came back saying: 'You must forget all about this subject, completely forget it, act as if you know nothing and tell your colleagues not to say a word'."

Secreted

The Guardian has asked Iraq's Defence Ministry on a number of occasions to respond to this allegation – so far they have not responded.

Once in Iran the hostages passed through the border town of Mehran. They were moved between different locations in Iran for several months – including the holy city of Qom, where they were secreted in basements in the Zimbeilebad district, a mainly Iraqi area.

A former major in the Revolutionary Guard, who worked in the organisation for more than 14 years, has identified two other prisons where he believes the men were held.

The major, who cannot be identified and who now lives outside Iran, says one is an al-Quds camp near a salt lake about 25 miles from Qom and the other is nearer the Iraqi border.

"My contact works for al-Quds," the major said. "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. My contact also told me they stayed in the salt lake camp for a month ... they separated the hostages.

"Everything in the prison gave the impression that they are in Iraq, not Iran: the guards, the buildings and even the cars. The uniforms used by security are all Iraqi to give the impression that they are being held in Iraq."

The major said he had no doubts the Iranian regime directly planned the kidnapping.

"The kidnapping at the Ministry of Finance was done was led by the Republican Guard al-Quds force," he said. "They work in cells: every cell is made up of five men. The head of each cell is Iranian, a member of the al-Quds force, and the four others are Iraqis. They prepare for every operation in the most minute of detail in Iran … they study tiny, minuscule things. They specialise in creating perfect disguises for their men who take part in operations like this."

Iranian involvement has also been confirmed by a senior serving Iraqi government minister with close links to Iran.

"This was an IRG (Iranian Revolutionary Guard) operation," he said. "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."

Although there appear to have been multiple locations where the hostages were held, the city of Qom became the centre for negotiation for the release of all the hostages.

It was where negotiators were sent to try to secure the release of the men; it was also where videos of the hostages were handed over for eventual broadcast.

According to the Guardian's information the chief negotiator, Sami al-Askari, would fly from Baghdad into the airport at Mahaabad. From there he would be driven to meet the leader of the kidnappers in Qom.

The man who held the life and death of the five Britons in his hands in Qom was a senior leader for the Righteous League. Trained and funded by al-Quds, he was one of their main operatives in Iraq.

He and his deputy are believed to be the only men who visited the five Britons while they were in Iran.

In the beginning the kidnappers made contact with the Hostage Working Group appointed by the Foreign Office every two or three weeks but, according to documents from the men's employers, BearingPoint and GardaWorld, the contact then tailed off.

The problem for the men was that once the Foreign Office became involved in the kidnapping, the option of paying a ransom had to be ruled out according to official government policy.

But the kidnappers didn't want more money. They wanted concessions, including withdrawal from Iraq from the British and American governments. They also wanted the release of the man running Iran's military operation in Iraq, Qais al Khazali.

One Foreign Office source said: "The problem with dealing with the Iranians is that, with them there is always 'another demand'."

According to one of the leaders of the Righteous League who spoke to the Guardian the negotiations did not go well.

The autopsy reports on the bodies of three of the men discovered that Jason Creswell and Jason Swindlehurst, both of whom were former paratroopers, had been killed some time between March and May 2008 and their bodies buried at secret locations in Iraq.

"We killed them because we said we would kill them if our demands were not met. And we have lived up to our promises," the Righteous League leader said in June, just before the first two bodies were found.

An Iraqi intelligence source said: "The first four were killed soon after [the kidnapping]. They had no use for them. They are trouble. The computer expert was the valuable one."

The government minister, who has had direct contact with the leader of the kidnappers in Qom, also said that he believed that Swindlehurst, Creswell, Maclachlan and McMenemy were executed by their captors because they were "just bodyguards".

"In the Middle East those kind of men are regarded as mercenaries, they are disposable. Peter Moore was seen as the valuable catch.

"It was clear to the Britons when they got the bodies that this was an execution. The men had their hands tied in front of their bodies and had been shot," said the minister.

"Peter Moore was the golden egg," according to Abu Rahman, an Iraqi journalist who made contact with the kidnappers on behalf the Guardian.

According to the interview carried out by Guardian films with Kuther, they initially had been tasked with spying on foreign consultants working in the ministries. He said Peter Moore was targeted because he was a computer specialist: "We had detailed information about him and the work he was doing. We had planned to kidnap him in person and we knew that there was another man who was an expert too – but he was out of our reach."

The events that led to the kidnappings began shortly after the US president, George Bush, ratcheted up the pressure on the Iranians with a televised address to the world warning Iran to stay out of Iraq and pull back their proxies.

Documents

Iran responded with an unprecedented attack on a US base in Karbala nine days later. The attack had the hallmarks of the al-Quds kidnap section of the Revolutionary Guard, according to the major who spoke to the Guardian.

The Guardian has also obtained documents released by the US military under the Freedom of Information act.

The documents reveal how the al-Quds operatives' English was so perfect that the Iraqi guards at the checkpoints "were convinced the attackers were American".

When the attackers entered the compound they shot dead Private Millican. Four other US soldiers were kidnapped and spirited away in US-style vehicles. They were tracked down by US helicopters but before evading capture they shot dead their handcuffed captives.

Tit-for-tat incidents continued with the arrest by the US of Khazali and his brother in Basra in March that year. Khazali is alleged to have been the mastermind behind the Karbala killings, and also the appointed head of all Iranian special groups in Iraq.

The next move – the kidnap of the Britons from the Ministry of Finance – by the Righteous League and the al-Quds force was to be decisive and was designed to achieve two aims.

The first was to halt the project Moore was working on. The sophisticated computer system being installed would have exposed any corrupt practices in the Ministry of Finance .

Judge Radhi Hamza al-Radhi, former commissioner of the Commission of Public Integrity in Iraq, testified to the US Congress on 4 October 2007: "The cost of corruption that my commission has uncovered so far across all ministries in Iraq has been estimated to be as high as $18bn." He had fled to the US in August 2007 after his family's home was targeted in a rocket attack.

Vance Jochim, who was the chief auditor and a US adviser for the Commission of Public Integrity based at the US embassy in Baghdad, has previously told the Guardian: "The new system would provide more transparency and accountability over the oil and other revenue handled by the finance ministry" – which he said had been resisting its implementation for nearly two years.

Eight months after the kidnappings, the only other location with a full record of the Iraqi government's financial transactions and records of possible financial misconduct – Iraq's Central Bank – was destroyed in a fire. The subsequent investigation found that it was arson.

The intelligence source said: "Many people don't want a high level of corruption to be revealed. Remember this is the information technology centre, this is the place where all the money to do with Iraq and all Iraq's financial matters are housed. The centre is linked to the Americans and all the money transfers. Everything, right down to the last penny, is in that centre."

But if one aim was to avoid detection of corruption, the second aim was to bargain for the release of the Khazali brothers. In the end that was achieved, although not before the four bodyguards surrounding Peter Moore had been killed.

Robert Baer, ex-CIA operative and author of several books, says the Iranian tactics are familiar and have changed little over the years.

"High-profile kidnappings have always been part of the Iranian tactics. In Lebanon the proxy groups they used to carry out the kidnapping were called 'Islamic Jihad' – in Iraq these days they call them the 'Righteous League'. The name doesn't matter – it is Iran who is behind them."

But despite there being a final deal to release the only living hostage, questions have been asked about the tactics employed by the British authorities throughout the ordeal.

According to a confidential document released by the five men's employers, the media plan endorsed by the Foreign office was "for minimal exposure". The companies employing the men also said they had pursued a policy of encouraging the families to keep a low profile because it was in the best interests of the men.

Over the months the profile of the hostages gradually diminished.

Spiritual leader

Since the kidnappings, the Foreign Office has never confirmed that they were taken to Iran and will say only that they didn't know where they were. The official line has always been that: "Her Majesty's Government policy is not to facilitate or pay ransoms or make substantive concessions to the hostage takers."

But the Guardian has learned that approaches were made to the Iranians through the British Embassy in Tehran - though not directly to the supreme leader, Ayatollah Ali Khamenei, who is the spiritual leader of al-Quds force.

Iraqi intelligence sources have also claimed that British intelligence – one of the best intelligence services operating in Iraq – knew that the five Britons had been taken to Iran.

"The British are the best, they know exactly what is going on." So why then did they not approach the spiritual leader and ask for the release of the five men?

"Deals had been done with Iran for the release of the British sailors – so why couldn't a deal have been done for our guys?" asked the father of one of the hostages, Denis McMenemy, a 57-year-old former paratrooper who works nights stacking shelves at Morrisons in Dumbarton.

Baer agrees : "The Americans have failed every single time, on every important incident between Tehran and Washington.

"And so has Britain, by not talking to the Iranians, by not establishing a channel to Khamenei's office. Not through the foreign ministry, not through the embassy, but to Khamenei's office, it's a diplomatic failure."

Iraq's national security adviser, Mowaffak al-Rubaie, who had been in direct contact with the kidnappers in Iran also agrees: "The kidnapping is a very complicated issue in the region.

"The other thing the Foreign Office could have done was to talk to the regional players in the region. But the modus operandi would have never been reaching out to Iran. It was a taboo you should avoid."

Over the last year, three funeral processions have wound their way through the streets of Wales, Scotland and England – carrying the bodies of Jason Creswell, Jason Swindlehurst and Alec Maclachlan. A fourth will take place in Glasgow for the funeral of Alan McMenemy.

In the end only Peter Moore made it out of Iran alive.

For Kuther, the question of compassion doesn't come into the calculus of kidnapping.

"I don't mean to hurt the families, but they knew there was a risk when their sons went to occupy another country. What they were afraid of has happened. It has happened to their five sons."