The hacking ring allegedly at the centre of thelargest-ever identity theft breach last week was also involved in cracking a network of Citibank-branded ATMs located in 7-Eleven stores and operated by a third company, a US law enforcement source claimed.

While hacking credit card data is common, attacks on ATM systems are much rarer and cause for greater concern because of the access they offer to large sums of money. Banks have said very little about their losses to cyber-thieves, but federal authorities say the figures have been growing rapidly.