While prosecutors are aiming to bring at least one currency case this year, the heavy workload could delay action until early next year. The pace also could stall as prosecutors seek to coordinate with the Commodity Futures Trading Commission, Mr. Lawsky and federal banking regulators.
In Britain, however, regulators are nearing a settlement with several banks in the currency case. The Financial Conduct Authority of Britain met last month with six banks — Citigroup, JPMorgan,Barclays, UBS, the Royal Bank of Scotland and HSBC — to discuss the contours of a collective settlement that it plans to announce this fall.
Those banks are not necessarily the most culpable, but rather the ones most willing to reach a settlement. While American prosecutors have not ruled out joining a global settlement, lawyers said, such a move appears unlikely.
Altogether, the British regulator could collect fines that total up to $3.3 billion, people briefed on that settlement said. Of the six banks, one person said, the size of Citigroup's payout is expected to fall in the middle.
Banks are eager to put the case behind them as they prepare to submit their capital plans to the Federal Reserve. Under the Fed's rules, the banks must set aside enough cash to cover a potential settlement, which can become an expensive guessing game without clarity from prosecutors.
Read MoreSenior banker in UK pleads guilty in Libor probe
At Deutsche Bank, facing both Libor and currency investigations, there is growing momentum to resolve at least one of them. In the Libor case, prosecutors have begun to coordinate with the bank's American regulators, including Mr. Lawsky, about the fallout from a potential guilty plea for the bank or one of its subsidiaries, lawyers said. That planning reflects a desire to criminally punish the bank without imperiling its ability to operate in the United States.
At their core, the investigations into Libor and currency trading center on suspicions that banks manipulated the benchmarks for their own gain. In Libor, a measure of how much banks charge one another for loans, several banks submitted false rates to benefit their trading positions.
The foreign exchange inquiry has pointed to a more complex scheme to fix currency prices and game the market. Authorities suspect that banks, using information gleaned from their clients, collaborated to flood the market with orders just seconds before the so-called 4 p.m. fix, which serves as the benchmark for foreign exchange rates. The aim in part, authorities suspect, was to drive up the price of, say, euros before selling them to clients at an inflated price.
Traders at competing banks met in private chat rooms. Some traders became so cozy that they earned the nickname "the cartel" and "the bandits club."