Barclays Plc's settlement of about $451 million with U.S. and U.K. regulators last week offered the first glimpse of what banks may have to pay to resolve a global probe of interest-rate manipulation. The question now is who's next.

The two-year investigation, which involves regulators on three continents, has touched as many as 18 financial institutions that help set London and Tokyo interbank offered rates for dollars, euros and yen. That number includes as many as 12 firms that have fired or suspended traders in connection with related internal probes of whether their employees tried to manipulate the rates known as Libor and Tibor.

The span of the investigation has prompted at least one regulator to ask for more time, according to a person with knowledge of the matter. The U.S. Commodity Futures Trading Commission recently sent out so-called tolling agreements in which the banks would waive their right to claim a lawsuit should be thrown out if a five-year statute of limitations has elapsed, said the person, who asked not to be identified because the agreements aren't public. CFTC spokesman Steven Adamske declined to comment.