For those of you who forgot this is why the good Senator is vulnerable to such slings and arrows:
Mitchell, a former senator, is a director of the Red Sox. Mitchell does not own a portion of the team, although as a director he has future equity rights if the team is sold.
I don't believe that a guy of George Mitchell's stature is going to go leaking this kind of stuff in order to help the team from which he draws a salary and whose sale could one day make him a boatload of money. It just seems far-fetched, not to mention risky. Mitchell presumably has some sensitive grand jury information in his possession now, so if he is discovered to be a leaker, he could face some heat from law enforcement, and the guy is just too smart and sensible to do that.
That said, just because you're not actually exploiting a conflict of interest doesn't mean that the conflict isn't a problem. Mitchell is a lawyer by trade, and lawyers know that merely saying you don't have a conflict -- indeed, even demonstrating that you don't have an actual conflict -- isn't always enough. There is such a thing as the appearance of impropriety, and such a thing is bad because it shakes the confidence of those who have placed their trust in you, even if you aren't doing anything wrong.
It's obviously too damn late in the game to do anything about it now, but given Mitchell's relationship with the Red Sox, he never should have been chosen as the dog to lead this hunt.