THE wedding guests arrived in black limousines to see a British secret agent marry his US government lawyer bride, surrounded by the strictest of security.

From the grand 19th-century Evermay mansion, where the ceremony took place, the guests had commanding views of America's power base, Washington, DC.

It is a city where former intelligence operatives and military men mix warily with politicians and power-brokers, looking for lucrative government security contracts.

Among the guests at the wedding were a former CIA station chief and a security adviser to Barack Obama. The best man had once been special operations marine colonel.

The guests were some of the best-informed people in the capital. Yet none knew that the wedding was a sham, the priest was an amateur actor and Richard Halligen, the groom, was an imposter.

Halligen, 50, is better known as Kevin Halligen in Britain (or more precisely Halligan with an 'a', according to his birth certificate).

The wedding was part of a illusion that has seen him take in some of the most senior figures in the intelligence world on both sides of the Atlantic with a mixture of charm and trickery.

On the way he has made considerable sums from the Madeleine McCann fund and more than '1m from a deal involving a company accused of dumping toxic waste. He has left a string of creditors behind. His debts are said to amount to more than '3m.

Halligen is now on the run after last being spotted with a girlfriend at the Royal Crescent hotel in Bath.

His pursuers include the former head of undercover operations for the UK police, a City lawyer, a Washington lobbyist, his former bride and a former head of the SAS, who blames himself for helping to launch Halligen into the world of intelligence and security.

The United States justice department, on behalf of the FBI, has issued an indictment seeking his arrest for an alleged '1.2m fraud.

The secretive nature of the security and intelligence community provided the perfect cloak for the talented Mr Halligen. It is a world where people do not talk openly about their past exploits, because they are frequently matters covered by the Official Secrets Act.

A Dubliner by birth, Halligen came to England in the late 1970s and held a series of consultancy jobs as an electronics specialist. His first spell as a director was for his girlfriend's catering company in Surrey, from which he resigned in 2001.

However, the stories he spun to colleagues and girlfriends about his history were much more colourful. They say he claimed variously to have worked for GCHQ, MI5, MI6 and the CIA, or at least implied as much.

In his r'um'he claimed to have worked on government defence projects for more than 20 years. 'Kevin,' he wrote of himself, 'has operational experience in Northern Ireland and the Middle East and retains close links with special projects, special forces and the international government security community.'

He also claimed to have contributed to a high-level report assessing Britain's resilience to terrorist attacks. However, this, like his claims to have briefed the Pentagon on Iraq, was at best fanciful.

His first entry into the private security business was as technical director for the Inkerman Group, a company set up by Gerald Moor, an ex-army intelligence officer. The job ended abruptly in 2003 after Halligen drank Moor's stocks of champagne and 'irreplaceable' burgundy while house-sitting for a couple of weeks.

At the same time, the company vetted Halligen and found worrying anomalies. 'Nothing seemed to check out. He had two different birth mothers, according to the forms he had filled out,' said Moor.

Once in the security world, however, Halligen began forging business contacts and friendships. A key to this was his membership of the exclusive Special Forces Club in Knightsbridge, central London.

Membership is usually open only to those who have served in the UK's secret services or special forces, but his name was nonetheless put forward in 2002 by two of the club's staunchest members: Major Donald Palmer, its then chairman, and Major-General John Holmes, a former commander of the SAS and former director of special forces.

Both men ' who knew Halligen through business ' now bitterly regret helping him. 'We were all taken in,' said Holmes, who now devotes considerable amounts of his time to finding Halligen.

'I feel partly responsible because I introduced him to people, my friends, some of whom are now owed money. I just want to see that he is brought to book.'

Using his contacts, Halligen set up his own companies. His initial venture Chimera (dictionary definition: 'vain or idle fantasy') was short-lived, and Halligen formed Red Defence International in 2004.

It was a crisis in Ivory Coast in September 2006 that was to transform Halligen and his company's fortunes.

Claude Dauphin, the president of the Dutch company Trafigura, had been seized with another executive in the African state. They were accused of dumping toxic waste near the country's largest city, Abidjan, which had poisoned thousands of people.

Halligen's company was paid '460,000 a month to identify the key power brokers in Ivory Coast and negotiate the executives' safe return. This did not turn out to be enough, however.

By now Halligen was operating from plush offices in Washington and, at some point, had acquired a defence department security pass. He used the cash from Trafigura to hire the services of some of the most powerful private security organisations in the city.

By January 2007, there was real concern. The Christmas deadline for the release of the two executives had passed and their families were becoming desperate.

Halligen met the Trafigura's British lawyer, Mark Aspinall, at a Washington hotel and asked for an extra '1.2m for a lobbying campaign to persuade the US government to intervene in the dispute.

Aspinall's London law firm was given the money by Trafigura and paid it into a personal bank account that had been set up by Halligen in Washington.

The money went into his account on January 10 and the following day '1m went out to pay for Halligen's palatial new home in Great Falls, Virginia.

The two Trafigura executives were freed the following month when the company paid '120m to the Ivory Coast government.

The deal had little to do with Halligen; but the company was so relieved that it did not inquire how he had spent the money intended for lobbying.

Aspinall, meanwhile, had struck up a friendship with Halligen during the crisis. When Halligen asked him to invest in Oakley International, his new Washington company, Aspinall handed over '300,000 and later made a personal loan of another '150,000. He has not seen a penny of that money since.

Halligen is thought to have made '1m in profit from the Trafigura work on top of the money that he spent on his home. He spent thousands of pounds staying at the Willard hotel in Washington ' where he met his 'bride', Maria Dybczak, a trade lawyer for the commerce department, when he accidentally stepped on her dress.

Dybczak has spoken to The Sunday Times but did not wish to be quoted in the article.

Halligen had told Dybczak that he had previously worked closely with MI5 and MI6. Sometimes he also claimed, for effect, that he had met her in Bosnia years earlier. He did not tell Dybczak that he already had a wife in Britain whom he had married in 1991 but never divorced. .

His '360,000 wedding to Dybczak, in spring 2007, was the perfect opportunity to confirm his entry into the Washington elite. Just 40 hours before the ceremony was due to take place in Washington's most expensive home, Halligen dropped his bombshell.

He told Dybczak that his spy masters in Britain would not allow his name to be made public on a marriage certificate. 'He said he was in ops so black that he could not allow his name to be on anything,' said a source close to Halligen.

The couple persuaded the catering manager, who was the director of a local theatre group, to play the role of the priest.

When Halligen slipped the '120,000 ring on his new 'wife's' finger, the scores of powerful guests had no idea.

One of the guests was Andre Hollis, a lobbyist who became chief executive of Halligen's Washington company. 'It was like a global intelligence debutante ball,' he said. 'And nobody knew it was fake.'

Not even the best man, Colonel John Garrett, a defence lobbyist for the blue-chip Washington law firm Patton Boggs, was let in on the secret. Nor was the most powerful guest in the room, Noel Koch, a security expert who has now become a deputy undersecretary in the defence department.

He said: 'We found out later that it was not a real wedding. The priest was an actor.'

Koch says Halligen was a curious character: 'He used to be difficult to understand because many of the conversations were sotto voce. It was like we're all spies together and the walls were listening.'

Also on the guest list from England were Palmer, Aspinall and Henri Exton, a former national head of undercover operations for the police.

It was Exton's expertise that was to be used in the hunt for Madeleine McCann.

Madeleine had been missing for a year when Brian Kennedy, the millionaire philanthropist who had invested heavily in a fund to find the little girl, contacted Halligen's firm via Exton.

At the initial meeting, Halligen seemed impressive. He offered the McCanns undercover surveillance and intelligence gathering in Portugal, as well as promising to provide satellite imagery and details of telephone traffic from the night Madeleine disappeared.

The contract was agreed with Oakley International, and the money was paid into the company's American account through a middle company called Housing Agent Holdings.

Former colleagues say Halligen was out of his depth and had no experience of such investigations. So the detective work on the ground was done by Exton and other contractors, who produced a series of reports pointing to new leads.

Halligen was supposed to provide the technical data such as the satellite imagery from his contacts in Washington. 'As far as I am aware,' said one source, 'all he came up with was a Google Earth image.'

The McCanns themselves were increasingly concerned about Halligen. 'He had this sense of cloak and dagger, acting as if he were a James Bond style spy,' said a friend close to the family. 'The McCanns found him hard to deal with, because he was forever in another country and using different phones. He promised the earth but it came to nothing.'

The contract came to an end in October last year and was not renewed. Halligen's company had received '500,000, but the contractors who did the work are owed more than '300,000. Cheques bounced despite Halligen's promises.

Exton, who was more than '100,000 out of pocket, switched his investigation into Halligen himself. As a result of his initial inquiries, he approached Aspinall, who was also concerned about his investment.

In October last year, as the pressure mounted, Halligen decided he needed a holiday in Rome. He told Dybczak that his 'former employers' in London had leaked details of his undercover operations in Northern Ireland and he needed to go into 'hiding'.

While texting Dybczak to say he loved her, he flew first-class to Rome with a new girlfriend and her dog, paid for by his company. They settled in the five-star Cavalieri hotel and enjoyed its Michelin three-star restaurant. The '12,700 bill was paid for on his company credit card.

Back in Washington, it was dawning on Hollis that nobody had been paid and the Madeleine money had gone. The '50,000 he invested in the company had also evaporated.Exton and Aspinall called in a forensic accountant to look at the books and the results were alarming.

In just over a year Halligen had skimmed off '600,000 from the company for his personal benefit. More than '150,000 had been spent on improvements on his new house. Payments to labourers, electricians and plumbers were all itemised.

After the first tranches of the Madeleine fund money went into the account, Halligen withdrew more than '130,000 for his personal use. The payments were often disguised as transfers to Dybczak, but she has confirmed the money was for his benefit.

Halligen is now believed to be back in Britain travelling with an old girlfriend. Dybczak has not seen him since he left Italy. He owes her '45,000, and her parents have not been paid back '170,000 they lent him. Patton Boggs, his lawyers, are also owed cash.

Having amassed a file of evidence, Exton called in the FBI, which is now seeking the 'secret agent'.

Insight: Jonathan Calvert and Claire Newell