There is still a lot of work to do before hundreds of millions of dollars can be returned to customers of bankrupt MF Global , the woman leading the Commodity Futures Trading Commission's investigation said on Friday.

CFTC Commissioner Jill Sommers, who heads up one of several probes of a massive shortfall in money that was supposed to be segregated for MF Global customers, said in a statement that dozens of auditors, investigators and attorneys are continuing to work on the case.

But subsequent transfers of those funds are currently being traced, and there is still a lot of work to do before the CFTC will be in a position to identify the exact source of the shortfall and work to pay back customers, Sommers said.

Most customers have already been paid back about 72 percent

of the value of their accounts.

Sommers' statement came two days after she said the CFTC's investigation was far enough along the trail to be able to determine where customer money went.

We certainly don't want to lead anyone to believe we don't know what happened, Sommers, a Republican commissioner, told Reuters on Wednesday. We do know, and we see where all the transactions went.

At the time, Sommers said investigators were trying to determine which of the customer transfers were valid and which were illegitimate.

MF Global filed for bankruptcy on October 31 after it was forced to reveal that it had made a $6.3 billion bet on European sovereign debt, spooking investors and customers.

Regulators and others probing MF Global say a huge chunk of segregated customer funds is missing, but they cannot agree on the amount. James Giddens, the court-appointed trustee liquidating the brokerage, has estimated the shortfall at $1.2 billion. Earlier estimates by other regulators pegged it at $600 million, while CME Group, MF Global's front-line regulator, has put it at between $700 million and $900 million.

MF Global's former CEO, ex-New Jersey Governor Jon Corzine, resigned on November 4. In three days of testimony before three separate Congressional panels, he has said he does not know what happened to the missing money. CME has said the firm misused customer funds as it faced a liquidity crisis.

CME Executive Chairman Terrence Duffy testified to Congress that a CME auditor participated in a phone call during which an MF Global employee indicated that Corzine knew the firm used customer money to lend $175 million to its European affiliate.

CFTC Commissioner Bart Chilton on Friday said his agency should work with Congress to develop an insurance fund for customer losses and should alter rules to keep customer funds from being invested in foreign debt.

There are other steps that may be considered as well ... such as full segregation - both for futures and for swaps - in the case of insolvencies, Chilton, a Democrat, said in a statement.

Sommers added in her statement that if MF's brokerage transferred customer money to the MF parent in bankruptcy, customers ought to be able to recover funds from the parent.

The brokerage liquidation is In re MF Global Holdings Inc, U.S. Bankruptcy Court, Southern District of New York, No. 11-2790.

The parent company's bankruptcy is In Re MF Global Holdings Ltd, in the same court, No. 11-15059.

(Additional reporting by Christopher Doering; Editing by Gary Hill)