He was a quiet loner who left small-town Brittany to study risk management at a middle-ranking university, where both friendship and academic excellence passed him by. By yesterday, Jérôme Kerviel had won himself lifelong notoriety, identified by his boss as the sole trader responsible for the biggest banking fraud in history. Last night police detectives, assisted by a locksmith, broke into his abandoned apartment in the plush Neuilly-sur-Seine suburb of Paris.

Managers have described him as a Machiavellian genius who managed to outwit the toughest financial control systems and plunge France’s biggest bank and world markets farther into disarray. Yesterday, however, a more mundane portrait was being painted of the man who brought Société Générale to its knees; that of a fantasist and an unsuccessful financier left to flounder in waters far too deep for him.

One colleague said that Mr Kerviel, 31, had not taken a holiday for eight months. “He knew that, if he left his post, he’d be found out.”

His mediocre abilities and limited experience have fuelled suspicions that he was the visible tip of an iceberg of incompetence at SocGen, the trusted, familiar French high street bank for whom he worked.

When he walked into his lawyer’s office early on Thursday morning, he was pale, haggard and bewildered at the storm breaking around him as bank executives revealed the fraud that had cost it an unprecedented €4.9 billion (£3.6 billion).

At the end of a three-hour meeting, Mr Kerviel’s barrister told him that he faced a maximum sentence of five years in prison and a fine of €75,000 if found guilty of forgery. The shy, introverted trader is said to have reacted with shock.

A source at SocGen said that he was in denial. “He seemed disturbed, he didn’t talk of his losses, He lives in a world of his own which he has created for himself — a world where he has only made profits.”

The bank sent him for medical tests amid a rumour among staff that he could be suicidal. In Brittany, Mr Kerviel’s family and friends were stunned to hear Daniel Bouton, the SocGen chairman, denounce him as a “cheat, a fraudster, a terrorist”.

In their eyes, he was the studious young man who had got himself on to the first rungs of a promising career ladder at one of France’s best-known financial institutions.

Sylviane Le Goff, his aunt, said he was being forced to carry the can for blunders made by Mr Bouton and his directors. She said that Mr Kerviel’s mother, a retired hairdresser from Pont L’Abbé, near Brest, had left for Paris to be with her son “because he’s not in a good way”. She added: “This is an honest family which has done nothing wrong. The young man has always been serious and reserved. He must have been manipulated. He didn’t put any money in his own pocket.” His father, who taught apprentices boilermaking in a local training centre, died less than a year ago. In Pont-l’Abbe, Yvette Lepine said: “I can’t believe it’s little Jerome who lived just down the road. I’ve known him since he was a baby. It’s a great tragedy. I’m sure he didn’t do it on purpose. I’m sure he tried to make it right.

“His father died and about the same time he was dropped by his woman. He thought they had a future together but it’s all gone wrong for him. The two things happening at the same time must have been why everything went wrong.”

Thierry Mavic, the Mayor of Pont l’Abbé, said he was flummoxed by the accusations. “He was a calm, collected and thoughtful young man — a little reserved.”

Mr Kerviel stood for local council elections as a candidate for President Sarkozy’s centre-right Union for a Popular Movement in 2001, according to Mr Mavic, who said: “I asked him to stand on my list. He accepted. He was interested in a public life and politics.”

Although the young banker was not elected, his aura continued to shine in his home town. He was smartly dressed and good-looking, and seen as a success story. But the reality is that he was no golden boy. His earnings came to about £74,000 last year, small by the standards of traders in Paris.

His performance-related bonus in 2006 was only about £1,000, a pittance in his profession and an indication that his dealings were modest. This is perhaps unsurprising given his background.

After leaving the lycée in Brittany, he took a degree in finance at Nantes University, a distinctly low-key institution far removed from the prestigious French business schools that teach the high-flyers of the future. He then took a master’s degree in finance at Lyons University, where he specialised in the risk management that he so spectacularly ignored in his professional career. In Lyons, as elsewhere outside Pont L’Abbé, he left almost no trace. “No one can remember him at all,” one teacher said.

Mr Kerviel entered SocGen in the summer of 2000 as a back office employee to work on the control systems that he was later apparently to defeat. He became an assistant trader in 2002 and a trader in his own right two years later. But he lacked the flamboyance of many of his colleagues.

“He spoke little, and he’d answer questions with yes or no,” one employee said. Another said: “He was pretty introverted and seemed ill at ease. He wasn’t a show-off. He tried to please his managers but he didn’t have what it takes to be a trader.”

A member of the equity derivatives section, where Mr Kerviel worked, said: “He was just an average kind of person.”

Colleagues knew little of his private life, except that he had a girlfriend — some think that she became his wife — from whom he was separated. He barely talked about judo and sailing, the two hobbies mentioned in his CV.

At his €1,500-a month rented flat it seems that he was just as discreet. His neighbours said that they had barely exchanged more than a “bonjour” with him. “He spoke to my dog more than he spoke to me,” one said yesterday.

The 19th-century building is in a smart, busy street, but inside, the beige walls, fading blue carpet and dark corridors combine to form a tawdry, dispiriting atmosphere.

Mr Kerviel was absent yesterday and is thought to be staying with his brother elsewhere in the Paris region. Maître Elisabeth Meyer, his lawyer, denied reports that he had gone on the run and said that he was prepared to answer police questions over the scandal. When it comes, the interrogation is certain to be long and detailed as detectives seek to determine just what Mr Kerviel did, and why.

A well-placed SocGen source said: “He does not seem to have made any money for himself. If he had, we could have got it back. But it’s just gone up in smoke.”

Colleagues believe that he began bending the rules to hide his losses after a series of ill-considered decisions and then found himself drawn into a web of deceit.

The bank claims that he acted on his own to set up a “scheme of elaborate fictitious transactions” to place unauthorised financial bets totalling about €40 billion and cover his tracks.

But this version is contested by many French experts, who claim that it would be impossible for any trader, and especially one as inexperienced as Mr Kerviel, to have executed such a plot without help or, at the very least, a blind eye, from his managers.

Elie Cohen, an economist at the National Centre for Scientific Research, said: “The feeling in the trading rooms is that it isn’t possible for a single individual to have done that. It seems a little far-fetched.”

A SocGen employee who has worked there for ten years said: “I can tell you, even at the higher levels of management, no one believes the official story — it’s a pathetic cover-up.”

Blogs and editorials

“Who is responsible for the mad risk-taking which has won over the markets if it is not the banks and their directors? Who is responsible for the failure to control a French trader if it is not the banking commission chaired by Christian Noyer?” Libération editorial

“The chairman [Daniel Bouton] knows deep down that what is presented as a fraud has exposed the dangerous failings of the risk control system. It is astonishing that no alarms went off” Le Figaro

“What we have learnt from the Société Générale shows us that irresponsible money management is not only the result of mad traders in the United States” Le Courrier Picard

“The Société Générale is going to have to convince us that investing in it is not shooting yourself in the foot” L’Union regional newspaper

“I work at SocGen. I’ve been there for the last ten years. I can tell you, even at the higher levels of management no one believes the official story, it’s a pathetic cover-up. Bouton has nobody’s support, even that of the most loyal, and nor do any of the members of the board. Everyone is completely disgusted.” A SocGen staffer on Charles Bremner’s blog for timesonline

“He [Kerviel] is not only the most wanted man in France but a star on the web. His name has been searched 12,000 times on Google in the past 20 hours. Before the scandal he had five friends on his Facebook profile. Now he has 764” Halfday blog

“This is a serious failing of the Société Générale. It’s disgusting to lay all the blame on one trader” Posting on Le Monde’s website

“You’re a saint Jérôme. It’s a shame you didn’t lose 50 billion” The Jérôme Kerviel fan club site on Facebook

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