Officials in other states, including California, Florida and Virginia, have already joined suits against large custodian banks alleging they overcharged for their services. The banks named in the suits to date — Bank of New York Mellon Corp. and State Street Corp. — have denied wrongdoing.

North Carolina has not sued either bank. But a spokeswoman for State Treasurer Janet Cowell said via email late Monday that the agency has sought proposals from law firms "to evaluate any claims that the Department may have arising from foreign exchange trades executed by its custodian bank(s)."

What might make a North Carolina action unusual is that it could target both large custody banks, which so far have defended themselves separately. BNY Mellon has been the agency's custodian bank since 2007. Before that, custodial services were provided by State Street, said Treasury spokeswoman Heather Strickland.

Spokespeople for both banks declined to comment.

Strickland did not immediately respond to questions about why the agency is looking for attorneys now or how the decision might have been informed by the actions in other states.

Stickland sent comments in response to a questions about a bid invitation from her agency requesting proposals from law firms to review "and potentially litigate" forex claims. The firm chosen will evaluate potential claims on their legal basis, chances of success and the amount of recoverable damages, among other things, according to the request.

Traditionally, foreign exchange operations have been a profitable area for the custodial banks, more so than their traditional record keeping businesses. But even before the suits, competition and customer scrutiny had driven down prices.

Lawsuits so far have alleged the banks inflated their profits by charging customers higher prices than those at which they actually executed foreign exchange trades — often the highest prices of the day.

Broadly, the banks have responded they were charging customers for bundled services and that customers understood the arrangements.