In a statement, Well Fargo says a computer containing names, addresses, account details and social security numbers for a small percentage of its personal line and loan customers was stolen from the office of an analyst hired by the bank. Equipment was also stolen from other offices of unrelated businesses in the same building.

The bank stresses that no passwords or personal identification numbers (PIN) were on the files and no other accounts are affected.

Well Fargo says there is no indication so far that any stolen information has been misused and it is working with the Concord Police and the Northern California Computer Crime Task Force on the investigation.

Doreen Woo Ho, head of the bank's consumer credit group, says: "We're doing everything we can to make sure that those responsible for this crime are apprehended as quickly as possible and the perpetrators understand that the account data on these computers is going to be rendered useless to them."

Woo Ho says the bank is implementing additional security measures for affected customers. The bank is currently changing personal line of credit account numbers, providing free one-year membership of its credit monitoring service PrivacyGuard and adding a reporting alert to credit files.