A former MF Global executive says she expressed concern about the brokerage firm putting customers' money at risk four days before it filed for bankruptcy protection.

Christine Serwinski, who was the firm's chief financial officer for North America, says in testimony prepared for a congressional hearing Wednesday that she learned Oct. 27 of a "substantial deficit" in the cash kept by the firm in customer accounts as a cushion against potential losses.

Serwinski says the deficit didn't violate rules but made her uncomfortable.

Serwinski says she asked colleagues about it and they assured her the situation was "under control." Regulators weren't informed of a shortfall in customer money until Oct. 30.

MF Global filed for bankruptcy protection Oct. 31. About $1.6 billion of customers' money is still missing. 

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