Who broke the law by raiding customer accounts at MF Global Holdings MFGLQ +18.52% Ltd.? Investigators seem no closer to the answer than they were when the New York brokerage firm filed for bankruptcy exactly a year ago Wednesday, owing thousands of farmers and ranchers, hedge funds and other investors an estimated $1.6 billion. Their money was supposed to be stashed safely at MF Global, but company officials used much of it for margin calls and other obligations. The last, best hope for a breakthrough in the probe is Edith O'Brien, the former assistant treasurer at MF Global. Working in...