Code of Federal Regulations Title 43. Public Lands:  Interior § 43.10010.16 When to prepare

(a) An EA will be prepared for all actions, except those categories of action excluded from documentation or addressed adequately by a previous environmental document, or for those actions for which a decision has already been made to prepare an EIS. The purpose of such an EA is to allow the responsible official to determine whether to prepare an EIS.

(b) In addition, an EA may be prepared on any action at any time in order to assist in planning and decision making.