Updated

MF Global, the securities firm led by Jon Corzine, admitted using clients' money as its financial troubles mounted, a federal official says.

An MF Global executive admitted that to federal regulators in a phone call early Monday after regulators raised questions about the company's books, according to an official familiar with the conversation.

It is not clear where the money ended up or what it might have been used for, said the official, who spoke on condition of anonymity because he wasn't authorized to discuss an investigation by federal regulators.

Government rules require securities firms to keep clients' money and company money in separate accounts. Violations can result in civil penalties.

MF Global filed for bankruptcy protection Monday, after a big bet on European debt threatened to topple it.

The investigation of MF Global Holdings Ltd. is preliminary. A formal investigation by the company's main regulator, the Commodity Futures Trading Commission, requires a vote by its five commissioners.

It isn't clear whether the violations could lead to criminal charges. At a news conference, Manhattan U.S. Attorney Preet Bharara would not comment on whether a criminal investigation is underway involving the revelations at MF Global.

Earlier Tuesday, the head of the Chicago Mercantile Exchange said that MF Global had violated rules requiring it to keep clients' money in separate accounts.

Craig Donohue, CEO of CME Group Inc., which operates exchanges where derivatives are traded, said MF Global was "not in compliance" with requirements set by CME and the CFTC.

"While we are unable to determine the precise scope of the firm's violation at this time, we are investigating the circumstances of the firm's failure," Donohue said.

Derivatives are investments whose value is based on the value of some underlying asset. MF Global was one of the biggest players in the derivatives market.

CME Group is involved in the investigation because it regulates companies that trade on its exchanges. Government regulators empower companies that run exchanges to enforce trading rules. When serious violations are alleged, these companies and regulators both investigate.

The firm did not respond to a request for comment. Interactive Brokers, which was considering buying MF Global until the problems came to light, declined to comment.

Corzine, a former New Jersey governor and chief of Goldman Sachs, took over MF Global last year. He led MF Global to make more trades for the company's own profits, a practice known as proprietary trading. Proprietary trading helped turn Goldman into a trading powerhouse in recent years.

Under Corzine's leadership, MF Global bet $6.3 billion on debt issued by Italy, Spain and other European nations with troubled economies. Those bonds have lost value in recent weeks as fears have intensified that some European countries might default.

Regulators said in September that MF Global was overvaluing some of its European debt investments. It required the company to raise more cash, according to court papers filed on Monday.

MF Global reported its biggest ever quarterly loss last week, mainly because of losses on proprietary trading. Credit rating agencies downgraded the company's bonds to junk status. And business partners demanded that it put up more cash to guarantee its trades. The result was a cash crunch that forced MF Global into bankruptcy court.

MF Global is the first big Wall Street casualty of the European debt crisis, which has roiled financial markets for months. Greece can't afford to pay its debts without outside help. European leaders have been wrangling over the details of bailouts for Greece, Ireland and Portugal.

Europe's debt problems threaten the financial system because European banks hold billions in debt issued by Greece and other troubled countries. Losses on those bonds could topple the biggest European banks. MF Global's failure highlights that threat on a smaller scale.

The Securities and Exchange Commission and the CFTC have said they and other regulators were monitoring MF Global's situation for days "in anticipation of a transaction that would include the transfer of customer accounts to another firm."

The regulators said MF Global had reported "possible deficiencies" in client accounts, but they did not reveal that the company had diverted client money.

That proposed deal fell through after regulators and the potential buyer, Interactive Brokers, couldn't make the numbers add up. The discrepancies led to MF Global's admission at 2 a.m. Monday, the official said.

Trading of MF Global shares was permanently suspended on the New York Stock Exchange Tuesday afternoon.

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Associated Press writer Larry Neumeister in New York contributed to this report.