Due to...*ahem* a new development in the plot, I've decided to take this opportunity to revisit the Renya Karasuma case. The story of the mysterious rich guy is told is told in Files 300-302.
Keep in mind the translation I'm using is heavily flawed (terrible is more like it), so I may may be flat out wrong on some points. I'm assuming that all references to the "Crow Association" are mistranslations, and that the only party involved was a singular individual named Renya Karasuma.
He owned a mansion out in the backwoods of Japan. 40 years ago, he passed away at the age of 99. As he was dying he arranged a set of circumstances in which he was going to murder a whole bunch of rich people in the process. After death, his large collection of art was auctioned off in one big event. Two mysterious men showed up and gave everyone a leaf to roll up and smoke. It was mentioned as probably being marijuana, and everyone there started murdering each other under the influence. Then the two men made off with his treasures. Since it was stated that Karasuma was responsible for it all, it can be assumed that those two men were working for him.