A spokesman for Deutsche Bank also declined to comment but referenced the bank's statement in its second quarter earnings release, where it said it would "continue to cooperate" with regulatory investigations.
Spokesmen for Mitsui and Julius Baer, both said they would fully co-operate with the investigations.
News of the investigation comes as one bank moved closer to resolving precious metal trading investigations with global regulators, sources told CNBC.
Chief executive of Sharps Pixley, a London-based physical bullion broker, Ross Norman told CNBC there wasn't a "single gold trade that hasn't been picked over" by regulators in the U.S., Europe and now Switzerland.
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"It is a good, clean, efficient market," Norman said, adding that certain banks have been investigated "dozens of times" and "nothing untoward has been found."
Here we look at some of the details of the probe.