Commodity Futures Trading Commission investigators are poring over one million e-mails and instant messages as part of their price-manipulation probe of a swaps benchmark that helps determine interest rates on everything from annuities to bonds linked to skyscrapers.

Investigators preparing to interview bankers and brokers in the coming weeks are scouring the records collected under subpoena for any evidence that the world’s largest banks and ICAP Plc brokers rigged the ISDAfix swaps rate, said a person familiar with the matter, who asked not to be named because the probe is private. ICAP Chief Executive Officer Michael Spencer said last week that the company’s investigations have turned up no wrongdoing.

Electronic communications proved critical for regulators who fined Barclays Plc, UBS AG and Royal Bank of Scotland Group Plc more than $2.5 billion for rigging the London interbank offered rate, or Libor, the benchmark tied to $300 trillion worth of securities worldwide. Like Libor, ISDAfix is created by averaging submissions from banks rather than actual trade data.

Steve Adamske, a CFTC spokesman, declined to comment, as did Guy Taylor of ICAP.

“We have very strict rules for our staff who work on the dollar swap desk,” Spencer said on a May 14 conference call with reporters. “So far, nothing that we have discovered in our internal investigations gives me sleepless nights, and nothing that I’ve heard externally suggests ISDAfix has been tampered with.”

ISDAfix’s Reach

Companies and money managers in the $379 trillion swaps market rely on the ISDAfix benchmarks to value their trades. Banks use the rates in setting coupons paid on $550 billion of bonds tied to commercial real estate. The benchmark, which is set in six currencies, is used to price euro-denominated corporate bonds, and fluctuations in the rate help determine the performance of structured notes bought by wealthy individuals.

The CFTC issued subpoenas to current and former ICAP brokers, as many as 15 Wall Street dealers and to ISDA, Bloomberg News first reported April 8, citing people familiar with the matter. The U.K. Financial Conduct Authority also started an inquiry into how the benchmark swaps prices are set in British pounds, Bloomberg reported two weeks later, citing people familiar with that review.

Bloomberg LP, the parent of Bloomberg News, competes with ICAP in some businesses, including foreign exchange trading.

The banks that contribute to ISDAfix are Bank of America Corp., Barclays, BNP Paribas SA, Citigroup Inc., Credit Suisse Group AG, Deutsche Bank AG, Goldman Sachs Group Inc., JPMorgan Chase & Co., Morgan Stanley, Nomura Holdings Inc., Royal Bank of Scotland, UBS and Wells Fargo & Co., according to ISDA. Representatives of the firms either declined to comment or didn’t immediately return telephone calls.