A congressional subcommittee voted on Wednesday to issue a subpoena to a key employee of MF Global Inc, the futures-trading firm whose collapse in October triggered ongoing inquiries into the disappearance of more than $1 billion of customer money.

The employee, Edith O’Brien, served as an assistant treasurer in the firm’s Chicago office.

The decision to subpoena O’Brien means she will likely have to appear at a congressional hearing on MF Global to be held on March 28.

Her insights into the complex tangle of money transfers could shed more light on the final days of MF Global, as the firm, led by former U.S. senator and New Jersey governor Jon Corzine, struggled to meet its trading commitments and save itself from bankruptcy.

During those chaotic days, the firm appears to have tapped futures accounts belonging to its customers, funds that are required by law to be held strictly segregated from the firm’s own money, according to regulators investigating the firm.