The European Commission says many of the world's largest investment banks appear to have colluded to block attempts by exchanges to trade and offer more transparent prices for financial products known as credit derivatives.

The commission, the executive arm of the European Union, said Monday it has informed 13 banks — including Citigroup, Goldman Sachs, JPMorgan and Morgan Stanley — as well as the industry association for derivatives itself, the International Swaps and Derivatives Association, ISDA, of the preliminary conclusions of an investigation that began in March.

The Commissioner for competition policy, Joaquin Almunia, told reporters in Brussels that Deutsche Boerse and the Chicago Mercantile Exchange tried to break into the credit derivatives business between 2006 and 2009.

But "the banks acted collectively to prevent this from happening...because they feared it would reduce their revenues."

He said the banks will now have a chance to respond, but if the commission's suspicions are confirmed it "would constitute a serious breach of our competition rules."

The commission's investigation focuses on credit default swaps, often known simply as CDS contracts, which are essentially insurance that pays out when a company or country fails to honor a debt.

Indicating the size of the potential market distortion, Almunia said that at the moment there are 2 million such contracts outstanding, with a notational value of 10 trillion euros ($13 trillion).

The big banks have historically traded these contracts "over the counter" — that is, among themselves, releasing little information about trading prices to others who want to buy or sell them.

Almunia declined to say how much the banks might be fined if found guilty. He noted the banks themselves have been given some information about how fines could be calculated along with the official statement of objections they have been sent. But he said it was "too soon" to begin discussing fines publicly, given that the commission will only make a final determination as to whether the banks indeed broke antitrust rules later this year.

Other organizations that were sent the statement of objections were Bank of America Merrill Lynch, Barclays, Bear Stearns, BNP Paribas, Credit Suisse, Deutsche Bank, HSBC, JP Morgan, Royal Bank of Scotland, and UBS, as well as the data service provider Markit.