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Nineteen people suspected of stealing millions from online bank accounts have been arrested by police.

Detectives from the Metropolitan Police Central e-Crime Unit raided a string of addresses across London on Monday.

The suspects are alleged to be part of a gang that has stolen at least £6m in the past three months.

Police say that the international plotters used malicious code to capture bank log-in details that were held on personal computers.

The 15 men and four women arrested in Monday's dawn operation are aged between 23 and 47 and are being held at various police stations in London.

Detectives are questioning them on suspicion of fraud, money laundering and offences under the Computer Misuse Act. Two of those held were also arrested on suspicion of possession of a firearm.

International plot

Detectives say that thousands of computers are suspected to have been targeted by a gang who were using a trojan programme dubbed "ZeuS".

A trojan is a piece of "malware" that can appear as an ordinary application which is innocently or unintentionally installed by a user. Once in place, it secretly gathers information on the machine - and passes it back to its creator.

Detectives said that the gang had harvested log-in details operated by a range of major banks. Once the gang had the personal details, they transferred cash into accounts set up solely to gather the money before it was laundered onwards.

Detective Chief Inspector Terry Wilson of the Metropolitan Police said it was likely that the amount known to have been stolen would "increase considerably" as the investigation continues.

"We believe we have disrupted a highly organised criminal network, which has used sophisticated methods to siphon large amounts of cash from many innocent people's accounts, causing immense personal anxiety and significant financial harm - which of course banks have had to repay at considerable cost to the economy," he said.

"Online banking customers must make sure their security systems are up to date and be alert to any unusual or additional security features requested which is at variance with their normal log-on experience.

"Greater public awareness and education will make it harder for personal details to be compromised and for this type of fraud to be carried out."

DCI Wilson said that the operation had been helped by the "virtual task force" of police officers, computer experts and banking representatives who had shared intelligence and the latest research into online banking fraud.