Investigators probing the collapse of MF Global Holdings Ltd. are scrutinizing two money transfers made during the securities firm's final days in an effort to uncover what happened to $1.6 billion in missing customer funds.

Federal regulators at the Commodity Futures Trading Commission and the U.S. bankruptcy trustee for MF Global's brokerage unit are examining two separate transfers from customer accounts, including a previously undisclosed $165 million transaction, said people familiar with the matter. They said some of the investigators are poring over emails and records related to these and other money transfers.

MF Global collapsed into bankruptcy on Oct. 31, hurt by repercussions from an outsize bet its chief executive, Jon S. Corzine, made on bonds of troubled European countries.

Among those at the center of money transfers in the days leading up to the collapse was Edith O'Brien, an assistant treasurer at MF Global.

On the morning of Oct. 28, according to people familiar with the matter, Ms. O'Brien received a call from Mr. Corzine, who asked her to resolve a problem: The firm's bank account at J.P. Morgan Chase & Co. in London had run out of money, creating an overdraft.

After the call, Ms. O'Brien sent an email to about a dozen colleagues. "I need $175 mm sent immediately," said the message, a copy of which was reviewed by The Wall Street Journal.

Money to cover the overdraft was transferred to the London bank account later that day from MF Global's customer account, according to regulators and former MF Global officials.

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