The probe includes JPMorgan Chase, Citigroup, Barclays and Royal Bank of Scotland as well as the Zurich cantonal bank ZKB and private banking group Julius Bär, Comco said in a statement released on Monday.
"There are indications that the banks reached agreements about currency rates," Comco said, adding that it suspected the manipulation had affected "the rates of the most important currencies."
The banks are suspected, among others, of exchanging sensitive information and coordinating their buying and selling of currencies in a bid to impact the exchange rates.
The competition authority, which had already launched a preliminary probe into the matter last September, said it could not exclude that other banks and brokerages were involved in the suspected activities.
Credit Suisse was quick to react to the announcement, saying it was "astonished" by the allegations, especially since it had not been mentioned in the preliminary probe.
"These allegations are both inappropriate and harmful to our reputation," the bank said, adding nonetheless that it would cooperate fully with the investigation.
ZKB also said it was aware of the probe and that it would cooperate with authorities, but emphasised that it only played a very minor role in the currency exchange market.
Julius Bär also said it aimed to play a "constructive" part in the investigation.
The Zurich private bank said, though, that it had not come across any inappropriate activities during its own internal probe a few months ago.
Meanwhile, Switzerland's biggest bank, UBS, declined to comment on the investigation.
In its annual report however, the banking giant said it had received enquiries from different authorities and was cooperating with them.