Offsite storage specialist Iron Mountain lost the tapes, which included names and social security numbers for customers of America's City National Bank, on 28 April, and notified the bank last month. An unknown number of other US banks were also affected.
The backup tapes belonged to a Texas-based ISP that hosted a software application used by the banks. According to reports, they were lost while being transported from a data processing company to storage.
An Iron Mountain representative confirmed on Friday that "a small container of backup tapes" had been lost.
"The incident was investigated thoroughly by a federal law enforcement agency, which found no evidence of criminal involvement. Further, there has been no evidence of identity theft or that personal information has been at all compromised in this incident," said Iron Mountain in a prepared statement.
This incident comes is just the latest in a series involving the loss of valuable personal data. In March, Iron Mountain lost computer tapes containing personal info for about 600,000 current and former Time Warner employees.
Earlier this year, discount broker Ameritrade lost computer tapes containing data for about 200,000 customers, Bank of America lost tapes containing account info for more than a million federal workers, and Japanese bank Mizuho mislaid the confidential data of 270,000 account holders.