Is My Senator Pulling a Martha Stewart?

I don't think there is any hard evidence to prove that Senator Frist engaged in insider trading. Still, the situation is curious. Frist has long maintained that his owning an undisclosed, but presumably large, amount of HCA stock is not a conflict of interest. The senator has also been adament that, although his father started the company and his brother is HCA's former CEO and current director, he has no official relationship with HCA. Why, then, did Senator Frist suddenly sell his stock in HCA, coincidentally right before the stock fell 9%? (I suppose it is possible that Frist had no special knowledge of the stock's decline and that he severed himself from HCA for the sake of appearances before launching his 2008 presidential campaign.)