FMS is a collection agency primarily for student loans from the Dept. of Education.
They ARE NOT a government agency, no matter how the sound or what they say. They are governed by the Fair Debt Collection Practices Act and state laws.

If they threaten suit or garness wages, first check to see if the statute of limitations (when payment was last made or payments commenced) has expired for firing a suit against you.  It is typically 3-5 years, depending upon state.