U.S. senators plan to press regulators Thursday on whether they were asleep at the switch as now-bankrupt MF Global took on massive risky bets, and why hundreds millions of dollars in customer funds are still missing.

Commodity Futures Trading Commission Chairman Gary Gensler is one of the regulators called to testify before the Senate Agriculture Committee, in the first major congressional hearing about MF Global since it filed for bankruptcy on Oct. 31.

While Gensler will be able to answer some questions about the CFTC's MF Global probe, many inquiries are expected to be directed toward Jill Sommers, a Republican commissioner, who is overseeing the agency's investigation after Gensler recused himself in early November.

Mary Schapiro, chairman of the Securities and Exchange Commission, also is expected to testify.

``The public is still in the dark on basic facts,'' said Senator Chuck Grassley, a Republican on the Agriculture Committee. ``I hope the committee will be able to get some direct answers from Chairman Gensler and Chairman Schapiro.''

MF Global collapsed in late October after the firm was forced to reveal that it had made a $6.3 billion bet on European sovereign debt.

An effort to sell the firm failed, partially due to the revelation that hundreds of millions of dollars in customer money was not where it should have been.

Investigators have been scouring the company's books, described as messy and unorganized. The fund shortfall has been estimated at up to $1.2 billion by the liquidating trustee, but regulators believe that figure is too high.

Regulators and law enforcement officials are trying to determine what happened to the money and whether MF Global may have improperly mixed customer funds with the firm's own funds -- a major violation of industry rules.

The SEC is also conducting a broad review into how the firm conducted business.

The hearing on Thursday is the first in a series on MF Global, with the firm's former chief, Jon Corzine, called to testify at Senate and House hearings scheduled for later this month.

Originally the Thursday hearing was supposed to examine how each agency is doing in implementing last year's Dodd-Frank Wall Street overhaul law.

``I have critical questions about Dodd-Frank implementation which is what the hearing is about,'' Senator Pat Roberts, the panel's ranking Republican, told Reuters. ``However, given the extraordinary situation with MF Global, I plan to ask Chairman Gensler about his recusal and his oversight leading up to the bankruptcy.''

Gensler said in early November that he would not participate in the investigation of MF Global because he did not want to become a distraction or risk creating the appearance of a conflict of interest.

Gensler and Jon Corzine, who resigned as chief executive of MF Global last month, worked at Goldman Sachs Group Inc at the same time and held prominent positions. They both left the investment bank in the late 1990s.

MF Global had nearly a half dozen regulators policing various parts of the firm, including the CFTC, the SEC, the Chicago Mercantile Exchange, the Chicago Board Options Exchange, and the Financial Industry Regulatory Authority. But no single watchdog was responsible for the whole company.

Richard Shelby, the top Republican on the Senate Banking Committee, has called for the CFTC inspector general, Roy Lavik, to investigate the agency's oversight of MF Global.

Shelby, in a letter dated Nov. 30, asked Lavik for a detailed account of the CFTC's role in monitoring the firm and its customer funds. ``The fact that all of MFGI's customer funds cannot be accounted for raises serious public policy concerns,'' Shelby said in the letter.

He also wants Lavik to probe Gensler's decision to recuse himself. (Editing by Phil Berlowitz and John Wallace)