As if France’s second-largest bank didn’t have enough trouble. Thursday, after reporting that it had to write down an additional $3.02 billion worth of risky U.S. mortgage assets, Societe Generale SA also reported what may be the banking fraud of the century. And there are shadowy implications for the global banking industry.

Allegedly, a rogue bank employee illegally wiped out multiple billions worth of bank money. The bank blamed the $7.1 billion hit on a 31-year-old trader who supposedly acted alone to speculate in the future’s markets.

The Paris-based bank called it a case of “exceptional fraud” and said that the trader had set up “massive fraudulent directional positions in 2007 and 2008 beyond his limited authority.”

According to one senior manager at the bank, the rogue trader was of mediocre ability and not “one of our stars.”

Earlier last year, another French bank announced a similar case of trader fraud, albeit at a much lower monetary value. The problem this time is that this case of massive fraud comes when analysts were already questioning the bookkeeping of the banking sector in general.

Continual massive and unexpected write-downs from the largest banks across the United States and Europe have kept investors guessing as to the real creditworthiness of some of the world’s most prestigious financial institutions.

According to some sources, even the banks don’t trust each other. Consequently, interbank lending has severely constricted, which has led government-sponsored central banks to step in to keep the banking sector from collapsing.

Now, after Societe Generale’s announcement, a new question is being asked: If a lone rogue trader (who was somewhat ordinary) was able to cook the books for over a year to the tune of $7.1 billion at one of the world’s most well-known banks without anyone noticing, what other dirty little secrets could the banking sector be covering up?

Could there be more subprime and other related investments hiding in banks’ books ready to implode?

Judging by the falling share-price action of many U.S. and other banks, it seems like that might be a possibility, or at least some investors certainly seem to think so.