43 CFR 46.220 - How to designate lead agencies.
(a) In most cases, the Responsible Official should designate one Federal agency as the lead with the remaining Federal, State, tribal governments, and local agencies assuming the role of cooperating agency. In this manner, the other Federal, State, and local agencies can work to ensure that the NEPA document will meet their needs for adoption and application to their related decision(s).
(b) In some cases, a non-Federal agency (including a tribal government) must comply with State or local requirements that are comparable to the NEPA requirements. In these cases, the Responsible Official may designate the non-Federal agency as a joint lead agency. (See 40 CFR 1501.5 and 1506.2 for a description of the selection of lead agencies, the settlement of lead agency disputes, and the use of joint lead agencies.)
(c) In some cases, the Responsible Official may establish a joint lead relationship among several Federal agencies. If there is a joint lead, then one Federal agency must be identified as the agency responsible for filing the environmental impact statement with EPA.