The heat intensified Wednesday on the two most-powerful banks in foreign exchange, as it emerged that US regulatory officials were on a "fact-finding" tour of Citigroup's London office and that Deutsche Bank had suspended several employees as part of a sprawling investigation into practices by currency traders.

The officials from the New York Federal Reserve and the Office of the Comptroller of the Currency have been at Citigroup's office for a "couple of days," a person familiar with the matter said, and the bank is cooperating with their investigation.