Sen. Bill Frist defended himself Monday against insider trading allegations linked to his sale of

HCA

(HCA) - Get Report

shares.

In his first public comments on the issue, Frist said he "acted properly" in the transaction. The Tennessee Republican made the remark late Monday in a press conference. Federal prosecutors last week said they would look into Frist's sale of HCA shares just before the hospital chain warned in July of weak second-quarter numbers, driving its stock lower.

Monday's comments came as the new chief of one of the agencies investigating the sales,

Securities and Exchange Commission

Chairman Christopher Cox, recused himself from the probe. Cox, a former California representative and onetime contributor to a Frist election campaign, cited the need to avert what he called "any appearance of impropriety."

Frist has previously denied having any knowledge about problems at the company that prompted his sale, and has said he instructed a trustee to sell the shares to avoid a conflict of interest. A spokesman for Frist's office said last week that the Senate majority leader had "no information about the company or its performance that was not available to the public" at the time of the transaction. The spokesman stressed that Frist relies on a blind trust to manage his investments.

But he also conceded that Frist personally asked the trust in mid-June to sell off his HCA holdings and then left the trust in charge of carrying out those orders. Some observers have

questioned how blind that trust really is, given that Frist himself is apparently calling some of the shots.

On Monday, HCA shares fell 36 cents to $47.24.