WASHINGTON - Ending an investigation that clouded the tenure of former Senate Majority Leader Bill Frist, federal prosecutors have decided not to file insider-trading charges against the Tennessee Republican for his sales of stock in a family-owned chain of hospitals.

The U.S. attorney in the Southern District of New York and Securities and Exchange Commission staff last week sent Frist letters signaling they had closed their joint 18-month investigation. The letters essentially cleared him of wrongdoing.

Frist said in a statement that he "acted properly" and that his only reason for selling stock in his trust accounts was "to eliminate the appearance of a conflict of interest."

"I've always conducted myself according to the highest ethical standards in both my personal and public life," he said.

The stock probe had dogged Frist since news of his directive to sell his remaining HCA Inc. shares became public in the fall of 2005. Frist left Congress last year to "take a sabbatical from public life," he said at the time.

Frist's father and brother founded HCA, which grew into the nation's biggest hospital chain and last year became the subject of one of the largest ever private investment buyouts, valued at $33 billion.