There are still major questions surrounding the July collapse of Oklahoma oil-and-gas transporter SemGroup, but at the heart of the company's sudden bankruptcy are $2.4 billion in losses from traders betting oil prices would decline — smack in the middle of a run-up to $147 a barrel. There would have been plenty of business reason for SemGroup to be hedging, yet the stunning $2 billion margin call that knocked the firm into illiquidity definitely implies the strategy was more one of speculation, as does the odd fact that the company's then-CEO was personally responsible for about $290 million of the losses through his own trading company. A former director of the FBI has been appointed by the bankruptcy court to investigate the trades; most of the people involved so far haven't talked.