Two top Eurex investigators described to financial police earlier this month their warnings to Societe Generale in 2007 about unusual deals by rogue trader Jerome Kerviel, a source said Tuesday.

The head of Eurex's surveillance unit Michael Zollweg and a market analyst spoke to police April 10 regarding deals made by Kerviel, blamed for costing the French bank 4.9 billion euros (7.8 billion dollars) in losses, according to the source who is close to the investigation.

Confirming a report published in the French weekly L'Express, the source said the two men alerted Societe Generale several times in 2007 about Kerviel's trading behaviour.

After questioning the trader and apparently being satisfied by his answers, the bank tried to reassure Eurex, which is a Frankfurt-based European derivatives market, last November without complete success.

The bank did not act on a follow-up by Eurex after that.

Kerviel turned himself in to police on January 26, two days after the bank revealed the losses, and on January 28 was charged with breach of trust, fabricating documents and illegally accessing computers.

The 31-year-trader has maintained he acted alone but suggested his bosses knew he was dealing with huge sums of money and turned a blind eye as long as he was making a profit.

An internal bank inquiry found that Kerviel's unauthorised trading had not been detected because of his sophisticated techniques, but it also pointed the finger at internal audit and risk control failures.

Kerviel's supervisor has filed suit as a civil plaintiff in the case, a judicial source said Monday, but French prosecutors want the suit thrown out.