Estimates of the number of individuals range from three to six, although the spokesman would not confirm how many faced criminal charges.
The spokesman for the SFO, which has been working with the FCA and international agencies such as the US Securities and Exchange Commission, added: “Our investigation is in full flow and continues apace.”
Last year, during a plea and case management hearing of three known individuals Southwark Crown Court heard the SFO had written to 22 other people to say their alleged part in the Libor-rigging probe remained under investigation.
At the time, the SFO said none had been charged and some were yet to be interviewed.
So far, Terry Farr, 42, James Gilmour, 48, both former employees of UK interbroker dealer RP Martin, have been charged with conspiracy to defraud alongside ex-Citi and UBS banker Mr Tom Hayes.
Mr Hayes is expected to stand trial in January 2015; the other two have been given a trial date of September 2015.
All three have pleaded not guilty to the charges.
Last year, regulatory consultancy Bovill found the FCA had received more reports of suspected market manipulation over the past 12 months than before the Libor scandal broke.
Mark Spiers, head of wealth management at Bovill, said the number of cases had risen 43 per cent from 82 in the 12 months to the end of August 2012 to 117 in the year to August 2013.
That this jump came after Barclays was fined £290m in June 2012 for Libor fixing was “no coincidence”, he said.