Global investigations into alleged currency market manipulation intensified on Wednesday (January 15) as U.S. regulators descended on Citigroup's London offices and Deutsche Bank suspended several traders in New York, sources said.

The presence of Federal Reserve and Office of the Comptroller of the Currency officials at Citi's Canary Wharf office in the east of London this week comes after Citi last week fired its head of European spot foreign exchange trading, Rohan Ramchandani, following a prolonged period on leave, one source familiar with the matter said.

The suspensions of staff at Deutsche Bank in New York and possibly elsewhere in the Americas followed investigations into "communications across number of currencies," a second source said.

The FX case adds to Deutsche Bank's troubles after it had to pay a fine of €725 million, levied by European Union antitrust regulators for interest-rate manipulation in December.

These are the latest developments in the worldwide investigation into allegations that traders at some of the world's biggest banks colluded to manipulate the largely unregulated $5.3 trillion-a-day foreign exchange market, by far the world's biggest.

Deutsche and Citi are the two biggest players in that market, accounting for a combined 30 percent of that turnover, according to a Euromoney magazine poll. Deutsche has been the biggest FX bank for nine years running.

The Fed and OCC officials visiting Citi in London are at the "preliminary stage" of information-gathering and their presence is "independent" of Ramchandani's sacking, the first source added.

The OCC is an independent regulatory and supervisory bureau of the U.S. Treasury supervising nationally chartered banks, while the Fed oversees the holding company. Both declined to comment on the investigation.

An OCC spokesman said the OCC has an office in London to support its large bank supervision team there, so it would not be surprising that they have people visiting London branches of U.S. banks.

Presented by Adam Justice