Internet bank Egg worked with police to develop software that helped trace a gang behind what is thought to be Britain's first alleged attemped online bank robbery, it emerged yesterday.

Three people arrested following a six-month investigation into internet banking fraud involving the Prudential banking subsidiary have been released on bail pending further inquiries, the national crime squad said yesterday.

It also confirmed that other online banks may have been targeted by the alleged fraudsters. The arrests prompted experts to claim it was only a matter of time before criminals successfully carry out a "virtual bank raid".

Egg said its security systems were not breached and that there had been "absolutely no risk to customers' money whatsoever". But it confirmed that the alleged fraudsters "may have got away with a minimal amount". The fraud is said to have involved multiple bogus applications for loans and savings accounts.

Three people were detained after officers swooped on addresses in Buckinghamshire, Bedfordshire and Northamptonshire. Officers seized computer equipment as well as a quantity of cash and drugs in the raids on Tuesday.

Staff at the bank have worked with police over the past six months to help with the investigation. Tony Williams, Egg communications director, said: "This is not about hacking but multiple applications."

During the investigation Egg developed software that "rings an alarm bell" when a number of applications are made from the same address or PC.

The national crime squad said the possibility of attacks on other online banks came to light after studying the alleged fraudsters' computers.

Anthony Riem, partner at the "cyber law" specialist Philippsohn Crawfords Berwald, predicted that many more crime syndicates will attack internet banks, putting them at risk of losing millions of pounds.