When Joel Klein, the Justice Department’s antitrust chief, hired famed litigator David Boies to handle the trial of landmark antitrust charges against Microsoft Corp., he made no secret of it.

But Mr. Klein has also quietly turned to other private lawyers to assist with the Microsoft case as “consultants.” Among them, the lawyer with the most hands-on role appears to have been David B. Fein, of Stamford, Conn.’s Wiggin & Dana.