The rogue trader accused of single-handedly orchestrating the world’s biggest investment bank fraud is to be kept in custody for up to a further 24 hours, French judicial officals said today.

Jérôme Kerviel was being questioned by police last night about whether he had an accomplice.

The 31-year-old, blamed by French bank Société Générale for unauthorised trading said to cost it £3.7 billion and taken in to custody on Saturday, can be held until Monday afternoon. Under French law, after 48 hours in custody, he must either be released or handed preliminary charges.

Mr Kerviel was taken into custody after police searched his home and his office at Société Générale. Daniel Bouton, the beleaguered chairman of the bank, insisted yesterday that the trader had acted alone, like an “arsonist” who “burnt down a big factory”.

However, scepticism was growing this weekend about how he could have sidestepped so many controls designed to keep tabs on traders. Family members and finance experts called Kerviel a “scapegoat”.

Raymond Soubie, one of President Nicolas Sarkozy’s most senior advisers, said it was “very surprising” that one person had been able to invest a sum greater than the bank’s market capitalisa-tion of £25 billion without its knowledge.

The head of Société Géné-rale’s investment banking arm, Jean-Pierre Mustier, said Kerviel’s superiors had repeatedly missed opportunities to stop him after challenging unusual trades.

“In some cases he would tell them it was a mistake,” said Mustier, who conducted the bank’s interrogation of Kerviel. “He would convince them, for example, by cancelling the positions.”

Financial experts said that the £75,000-a-year trader would have needed extraordinary skills as a computer hacker to pull off such a large alleged fraud.

“It’s so unlikely that it was just one person,” said Xavier Timbeau, an economist. “He was gambling all of the bank’s assets.”

Kerviel’s aunt, Raymonde Kerviel, said: “My feeling is he is not capable of this and that there is more to it than meets the eye.”

City experts have cast doubt on the bank’s explanation of the alleged fraud. They believe the scale of Kerviel’s failed bets was so large that they could not have been cancelled over three days, as the bank has claimed.

Kerviel is thought to have been seeking a banking job in London before he was rumbled by his bosses in Paris.

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