The FCA said it first sent out letters to firms in April, before launching a formal investigation later.
Currency exchange rates are set on a daily basis by analysing actual trading volumes at leading banks during a short time window.
It is thought that traders could potentially influence exchange rates by pushing through large orders during the 60-second window to make a profit.
Even a small movement in exchange rates could affect the value of investments worldwide, including pension funds.
It threatens to engulf the industry in yet another embarrassing scandal at a time when many financial firms are still battling to restore their reputations following the Libor rigging revelations.
According to the Bank for International Settlements, global foreign exchange activity rose to 5.3 trillion dollars (£3.3 trillion) a day this year, from four trillion dollars (£2.5 trillion) in 2010.
London accounts for the bulk of currency trading, with 41% of global turnover in the market, followed by the United States, which has a 19% share, Singapore with 5.7%, Japan with 5.6% and Hong Kong with 4.1%.