The discovery that money could not be located might simply reflect sloppy internal controls at MF Global. It is still unclear where the money went. At first, as much as $950 million was believed to be missing, but as the firm sorted through its bankruptcy, that figure fell to less than $700 million by late Monday, the people briefed on the matter said. Additional funds are expected to trickle in over the coming days.

But the investigation, which is in its earliest stages, may uncover something more intentional and troubling.

In any case, what led to the unaccounted-for cash could violate a tenet of Wall Street regulation: Customers’ funds must be kept separate from company money. One of the basic duties of any brokerage firm is to keep track of customer accounts on a daily basis.