Marian Goldman

The amount of customer money missing from the collapsed trading firm MF Global may be more than $1.2 billion — double previous estimates — the trustee dismantling the firm’s brokerage unit said on Monday.

But the surprise finding, which caught regulators off guard, may be overstated, according to a person briefed on the investigation. Some regulators say they believe that the trustee double-counted $220 million that had been transferred between units of MF Global, this person said.

Still, the much higher number highlights the disarray of MF Global’s records and raises significantly the hurdle for tens of thousands of customers seeking to get their money back. The trustee’s estimate represents a significant portion of customer funds held by MF Global.

Regulators suspect that as investors and customers fled MF Global in the last week of October, the firm used some of the customer money for its own needs — violating Wall Street rules that customers’ money be kept separate from the firm’s funds. Much of that money may never return.

Now the challenge has grown for investigators trying to determine exactly what happened in those last frantic days. Just days ago, investigators believed that they were closing in on what they thought was about $600 million in missing customer funds, according to people briefed on the matter. Regulators were relying on estimates from the firm and the CME Group, the exchange where MF Global did most of its business.

But after weeks of reconstructing MF Global’s books, forensic accountants from Deloitte and Ernst & Young working for the trustee concluded that the account shortfall was much greater than originally estimated. Regulators have yet to verify the new numbers. While they are expected to raise their estimate above $600 million, it is unlikely to reach the trustee’s $1.2 billion figure.

Kent Jarrell, a spokesman for the trustee’s office, stood by that figure, but he noted that it was preliminary.

It is unclear what was behind MF Global’s original lower estimates. Some authorities chalk up the inaccuracies to the firm’s sloppy bookkeeping, and only slowly discovered additional holes in customer funds over the last three weeks.

The search for MF Global’s missing money has consumed a growing number of authorities, including the Federal Bureau of Investigation and federal prosecutors in New York and Chicago.

These inquiries have increasingly homed in on the theory that much of the customer money had left the firm, the people briefed on the matter said.

Regulators currently suspect that MF Global — at the time run by Jon S. Corzine, the former Democratic governor of New Jersey — improperly used customer money for its own purposes in the days before filing for Chapter 11 protection on Oct. 31.

Investigators are considering two possible situations. One is that MF Global used the money to meet trading partners’ demands for extra cash, which could come back. The other is that it was used to cover trading losses, which would mean that the money cannot be recovered.

MF Global’s management, however, has maintained that some of the money is still sitting at clearinghouses and banks, according to a person close to the company. Though they have not disputed that some of the money is gone, these executives think that other funds were trapped after the firm rapidly unwound more than half of its trading book as it was collapsing.

No one at MF Global, including its former chief executive, Mr. Corzine, has been accused of wrongdoing.

Representatives for MF Global, the CME and the Commodity Futures Trading Commission declined to comment.

The trustee, James W. Giddens, held a four-hour conference call on Sunday evening with staff members in New York City and Chicago to discuss the latest shortfall numbers, according to Mr. Jarrell.

Ultimately, Mr. Giddens — under pressure from customers demanding the return of their money — decided to provide his fullest update yet on the progress of his investigation.

“He felt duty-bound to say” that more money was missing, Mr. Jarrell said.

In Monday’s announcement, the trustee said that his office controlled about $1.6 billion in customer funds, but most of that was already earmarked to be paid out. The trustee said he was close to exhausting those funds. But the person briefed on the investigation said that the trustee would soon be able to tap more than $1 billion in customer money that is trapped in Harris Bank.

Beyond the shortfall in customer accounts, Mr. Giddens’s office said it did not have access to money that was held in foreign subsidiaries of MF Global, which are under the control of trustees in those countries.

“While the trustee will pursue them vigorously, it has been his experience that recovery of these foreign assets may take more time,” the office said.

In a separate move on Monday, MF Global’s estate requested court permission to appoint a trustee to oversee the winding down of the firm’s parent company. Such an authority would replace the company’s existing board. The trustee would be responsible for coordinating responses to regulators, among other duties.

MF Global is still running on about $8 million in remaining cash, and has yet to secure additional financing to support it through what will be a long bankruptcy case, lawyers for the estate said on Monday.

The fallout from the collapse of MF Global has renewed calls for tougher regulation of the futures industry, which has long relied on the principle that customer money is always safe.

While brokerages can use customer funds, they must put up sufficient collateral. Days before its Chapter 11 filing, however, MF Global was taking what amounted to free loans from its clients.

If federal prosecutors determine that MF Global intentionally tapped the customer funds, they could file criminal charges. But in a speech on Monday, David Meister, the C.F.T.C.’s enforcement chief, said that his agency need not show intent.

“You should know the commission takes the laws on segregated funds very seriously,” Mr. Meister said.

MF Global trustee’s statement