Deutsche Bank, BNP and Moore Capital staff in FSA raid
A total of 143 FSA employees took part in the operation
Deutsche Bank, the leading German bank, the French giant BNP Paribas and hedge fund Moore Capital are now known to have workers caught up in an FSA probe.
Their employees are among six arrested under suspicion of taking part in a long-running insider-dealing scheme.
The investigation is a joint venture between the FSA - the City's watchdog - and the Serious Organised Crime Agency.
The operation, which began in 2007, is the FSA's biggest operation against insider dealing.
It is also the first run between the two agencies.
Investigators raided 16 addresses in London, the South East and Oxfordshire, seizing documents and computers.
The FSA is probing private dealings by the traders, rather than trades for the firm.
The firms say are co-operating with the investigation.
It is not yet known where the other three involved were employed.
Suspected dealing ring
They are suspected of being involved in what the FSA calls "a sophisticated and long-running insider dealing ring".
The BBC's Robert Peston: "Extraordinarily embarrassing" for Moore Capital
It is alleged the city professionals passed inside information to traders, either directly or via middlemen.
The traders then allegedly acted on this information and made significant profits as a result.
The raid on Moore Capital was first discovered by the BBC.
A source said that some of the suspicious trading activities allegedly had been made using the arrested hedge fund manager's personal trading account.
A total of 143 FSA employees took part in the operation, together with officers from Soca.
"There can no longer be any doubt that the FSA is serious about cracking down on City crime, especially illicit trading in shares and securities when in possession of privileged insider knowledge," said BBC business editor Robert Peston.
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