The Securities and Exchange Commission (SEC) has launched a probe into whether MF Global Holdings made misleading statements about the $6.3 billion bets that sank the company, the Wall Street Journal said, citing people familiar with the matter.

MF Global, the futures brokerage which collapsed on Monday after risky trades on European debt, faces a shortfall of $633 million in customer funds, according to an estimate from CME Group Inc.

A source familiar with the matter told Reuters that regulators are still not sure where the money is, and why they can't find it.

MF Global maintains that any shortfall is the result of money stuck at banks that handled its trades or unintentional bookkeeping problems, the Journal said, citing people familiar with the matter.

The securities firm's trades are usually cleared on the same day. But as MF Global's situation became more dire, the banks held on to the funds, one of the people told the WSJ.

MF Global and the SEC could not immediately be reached for comment by Reuters outside regular U.S. business hours.

(Reporting by Sakthi Prasad in Bangalore; Editing by Muralikumar Anantharaman)

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