The SFO opened the investigation in July at the request of British politicians after Barclays was fined a record 290 million pounds ($462 million) for rate manipulation. Regulators across the globe are investigating claims banks altered submissions that were used to set Libor in an effort to benefit traders, or to appear financially healthier than they were.

The arrests could be temporarily delayed because of disruptions to the SFO's schedule caused by moving the prosecutor's offices from Elm Street to Trafalgar Square.

Egregious Attempts

Green said the agency is focusing on the most egregious attempts to manipulate Libor and other related benchmarks, with probes into firms, managers, traders and rate setters at lesser offenders coming later.

Regulators in the U.S. and U.K. are probing how derivatives traders and bankers who submitted interest-rate data colluded to rig benchmarks to benefit their own trades, and whether banks low-balled submissions in 2008 to hide their true cost of borrowing. Criminal probes by the SFO and U.S. Department of Justice are running in parallel with civil investigations being conducted by the DOJ's fraud division, the U.S. Commodity Futures Trading Commission and the U.K. Financial Services Authority.

Barclays spokesman John McGuinness, UBS spokesman Richard Morton and RBS spokesman Michael Strachan all declined to comment.

UBS and RBS are next in line to settle with the regulators, people familiar with the case have said. Edinburgh-based RBS has fired four traders following an internal probe.

UBS has fired more than 25 people after an internal review of interest-rate manipulation, a person familiar with the matter said. Robert Diamond, who stepped down as Barclays chief executive officer after the fine, said 14 traders were involved in wrongdoing at the bank.

The SFO has “hoovered up all the stuff from the FSA and loaded it onto our computers,” Green said. It has also received evidence from the U.S. Federal Bureau of Investigation and some of the banks.

The SFO is cooperating with a request from the U.S. Department of Justice for access to information from the U.K., Green said. That could include investigators from the DOJ sitting in on interviews that the SFO conducts with suspects, or having access to evidence the agency has gathered.

The request, which came under a mutual legal assistance treaty, or MLAT, was initially stalled while the SFO sought to get up to speed on the case. The DOJ submitted an amended request in recent months with “very substantial” information sought, Green said.

“Obviously when we first received it there was anxiety that execution of the request could mop up SFO resources,” he said. “We are anxious to execute it” and will “certainly” assist, he said.

Green said the agency, while working closely with the DOJ, is also competing to bring charges first in order to handle the prosecution of any British citizens in the U.K., reducing the chance of extradition.

The SFO's previous director, Richard Alderman, had declined to get involved in the case. Green took over as director in April.

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