State Street Corporation says computer equipment containing personal data belonging to staff and customers of its recently acquired Investors Bank & Trust (IBT) unit has been stolen.
The equipment was stolen from a facility belonging to a vendor engaged by IBT for legal support services.
IBT - which was acquired by State Street last year - had hired the vendor to review electronic records after receiving a request from federal regulators for information shortly after the acquisition.
A criminal investigation into the incident is under way. State Street has not disclosed what personal information was on the equipment or how many people are affected, although it has been reported that 5500 employees and 40,000 customers of the former IBT are being notified of the theft.
State Street says it will offer free credit monitoring where applicable.
In a statement on its Web site, State Street says it has analysed a copy of the data and found "no evidence to date to suggest that the data has been misused or that legacy State Street customers and employees are impacted".
The IBT data scare is the latest in a series of of security breaches over the last few months.
Last week it emerged that the Bank of New York Mellon has lost unencrypted back-up computer tapes containing the confidential details of around 4.5 million customers.
According to a statement released by Connecticut Attorney General Richard Blumenthal, a box containing between six and 10 unencrypted back-up tapes holding social security numbers, names and addresses - and possibly bank account numbers and balances - was lost in transit on 27 February.
Earlier in the month UK bank HSBC revealed it had lost a computer server holding the confidential personal details of around 159,000 customers from a branch in Hong Kong.