29 CFR §825.401

1. (a)A complaint may be filed in person, by mail or by telephone, with the Wage and Hour Division, U.S. Department of Labor. A complaint may be filed at any local office of the Wage and Hour Division; the address and telephone number of local offices may be found in telephone directories or on the Department's Web site.
2. (b)A complaint filed with the Secretary of Labor should be filed within a reasonable time of when the employee discovers that his or her FMLA rights have been violated. In no event may a complaint be filed more than two years after the action which is alleged to be a violation of FMLA occurred, or three years in the case of a willful violation.
3. (c)No particular form of complaint is required, except that a complaint must be reduced to writing and should include a full statement of the acts and/or omissions, with pertinent dates, which are believed to constitute the violation.