WILSONVILLE, OR -- A private investigation is underway on behalf of Mentor Graphics investors over possible breaches of fiduciary duty.

The investigation was launched following a back-and-forth by two major shareholders that were incensed over a recent decision by Mentor's board of directors to move up the date of its annual shareholders meeting.

The investigation, which is being undertaken by an undisclosed law firm, is looking into whether Mentor directors and officers or others breached or will breach their fiduciary duties in connection with a potential takeover or buyout. The firm seeks to ensure shareholders get a maximum return in the event of a sale of the company, says the Shareholder Foundation, a portfolio monitoring service and investor advocacy group.

Class-action lawsuits over companies in play are not unusual, and few result in findings of corporate malfeasance or material damages for shareholders.

Last week, a battle erupted between the company and shareholders, Carl Icahn, who directly or indirectly owns 14.7% of Mentor's stocks, and hedge fund Casablanca Capital, which owns another 5.5%, over whether Mentor changed the date of its shareholders meeting in order to head off the proposal of a dissident slate of directors.

On Friday, Icahn nominated his colleague David Schechter, plus Federal-Mogul chief executive Jose Maria Alapont, Exar director Gary Meyers, for Mentor's board.

Casablanca today announced it would propose three new board members for Mentor, including its founder Donald Drapkin, former NaviSite chief executive Arthur Becker, and 2KDirect chief executive Michael Barr.

Some media have reported Mentor has retained Goldman Sachs to help its takeover defense in a potential proxy fight with Icahn. Mentor has publicly denied the reports.