Federal investigators are nearing a decision on whether to grant a former MF Global executive immunity from prosecution, the FOX Business Network has learned.

As first reported by FOX Business, the possibility of giving former MF Global assistant treasurer Edith O'Brien immunity has reignited what was a moribund criminal case involving the  firm's implosion, including the loss of $1.6 billion in customer money.

The firm's bankruptcy trustee, James Giddens, is scheduled to give a detailed report today on the missing money.

O'Brien's attorneys have asked federal prosecutors for immunity in exchange for her full cooperation -- something criminal investigators have wrestled with since at least April, when FOX Business first reported the internal discussions. A federal grand jury has recently sent
subpoenas in the ongoing probe, though some law enforcement officials are skeptical that they have enough evidence to indict senior executives at the firm, including its former CEO, Jon Corzine, even with O'Brien's assistance.

A spokesman for the FBI, which is spearheading much of the probe, declined to comment.

People close to the investigation say federal prosecutors are now nearing a decision on whether to agree to "proffer" by O’Brien’s attorneys in which she would gain immunity in exchange for her cooperation in the ongoing criminal probe, people with knowledge of the matter say.

Despite misgivings about granting complete immunity, some federal law enforcement authorities believe O’Brien, as the firm’s assistant treasurer and one of the people in charge of MF Global’s funding, holds the key to whether senior officials at the firm, including Corzine, either gave direct orders for the transfer of customer funds to keep the firm alive during its final days, or knew customer funds were being used.

It’s unclear if prosecutors will grant O’Brien immunity or when they will make their final decision, but people close to the investigation say that several federal investigators are actively lobbying for a proffer deal.

Attorneys for O'Brien and Corzine didn't return calls for comment.

Charles Gasparino joined FOX Business Network (FBN) in February 2010 as Senior Correspondent.