The requests for information and witnesses come from a grand jury in Chicago and from the U.S. Commodity Futures Trading Commission's enforcement division, CME Group said in its annual financial filing with the Securities and Exchange Commission.

MF Global, run by former Goldman Sachs Group Inc. Chief Executive Jon Corzine, filed for bankruptcy on Oct. 31, and regulators are still seeking at least $1.6 billion of customer funds that went missing in the chaotic days before the firm's implosion.

The Department of Justice, the Federal Bureau of Investigation, the CFTC and the SEC are all conducting investigations aimed at finding the money and fingering those responsible, the CME said.

CME was MF Global's first-line regulator, in charge of auditing the firm. MF Global was also one of CME's largest customers.

"We continue to believe that we acted appropriately and that our actions do not give rise to liability," CME said in the filing. A CME spokeswoman declined to comment further.

CME also received a document request from the Securities Investor Protection Corp trustee handling the liquidation of MF Global, CME said. CME is also the target of several lawsuits stemming from MF Global's collapse, it said.

The trustee for MF Global's liquidation of MF Global estimates the shortfall in customer funds related to U.S. trading to be at least $900 million, with an additional $700 million in funds tied to trading on non-U.S. exchanges.

CME has been under pressure since MF Global's failure, as trading volume dropped and customer confidence dropped.