The former chief executive of Freddie Mac may face a civil action as the government ramps up an investigation of disclosure practices at the mortgage finance giant and its sister company, Fannie Mae, people briefed on the investigation said. The executive, Richard F. Syron, a former president of the American Stock Exchange and now an adjunct professor and trustee at Boston College, has received a so-called Wells notice from the Securities and Exchange Commission, an indication the agency is considering an enforcement action against him.