The Securities and Exchange Commission has completed its insider-trading investigation of Wall Street executive John Mack and big hedge fund Pequot Capital Management Inc. and is taking no action against them following a probe that prompted an allegation of political interference within the SEC.

Pequot Chairman Arthur Samberg sent a note to the hedge fund’s clients Thursday disclosing the agency’s decision to formally close the investigation without bringing charges against the company or its employees — as the SEC enforcement staff had recommended in October.