The hits keep coming for the troubled brokerage MF Global: The FBI is joining the investigation on how an estimated $700-900 million of client money went missing. The company, which filed for bankruptcy and is being delisted by the New York Stock Exchange, is headed by former Goldman Sachs chair and former NJ Governor Jon Corzine, who had hoped to turn it into a "mini-Goldman."

The Wall Street Journal reports, "It isn't clear if the discrepancy...was caused by money being diverted by company officials desperate to meet margin requirements or collateral calls as the financial situation became dire, these people said. Another possibility is that the gap reflects bookkeeping or accounting errors, as well as a delay in recording transactions in MF Global's books given the securities firm's far-flung trading and clearing operations." Either way, the company admitted it mixed its funds with client funds, which is against regulations.

Over the past few weeks, Corzine had been trying to sell the firm, which apparently ran into trouble with its bet on the European financial crisis after, as Reuters' Felix Salmon puts it, it "got involved in a complex liquidity-management trade, and that it didn’t have risk managers with the power or ability to cap the trade before it got too big."

Dealbook has a look at Corzine and his investing style:

Mr. Corzine, a lifelong trader with a large appetite for risk, recently reflected on his past experiences dealing with crises in William D. Cohan’s book “Money and Power, How Goldman Sachs Came to Rule the World,” a recent history of the bank.

“Until you’ve actually traded and had to deal with one of those come-to-Jesus moments with a bad position,” he said, “and you have to make the decisions about whether to eliminate it, hold it, reduce it — those kind of existential moments involving the people you work with and your firm, those are the kinds of things that really get your attention.”...

In May 2010, in his first investor call as chief executive of MF Global, Mr. Corzine discussed what could go wrong at the brokerage firm as it ratcheted up risk. “It will be mistaken judgments,” he said, “that go beyond limits, which I don’t intend to allow happen.”

Yet despite his confidence in risk management, Mr. Corzine’s message to traders was clear: Take more risks. “He was instrumental in pushing our firm forward with risk taking in every book, whether it was U.S. government bonds, currencies, or in repos,” said one trader. “Everything was full throttle go.”