United States Code/Title 28/Appendix/Federal Rules of Civil Procedure/Rule 24
(a) Intervention of Right. On timely motion, the court must permit anyone to intervene who:
(1) is given an unconditional right to intervene by a federal statute; or
(2) claims an interest relating to the property or transaction that is the subject of the action, and is so situated that disposing of the action may as a practical matter impair or impede the movant’s ability to protect its interest, unless existing parties adequately represent that interest.

(b) Permissive Intervention.
(1) In General. On timely motion, the court may permit anyone to intervene who:
(A) is given a conditional right to intervene by a federal statute; or
(B) has a claim or defense that shares with the main action a common question of law or fact.
(2) By a Government Officer or Agency. On timely motion, the court may permit a federal or state governmental officer or agency to intervene if a party’s claim or defense is based on:
(A) a statute or executive order administered by the officer or agency; or
(B) any regulation, order, requirement, or agreement issued or made under the statute or executive order.
(3) Delay or Prejudice. In exercising its discretion, the court must consider whether the intervention will unduly delay or prejudice the adjudication of the original parties’ rights.

(c) Notice and Pleading Required. A motion to intervene must be served on the parties as provided in Rule 5. The motion must state the grounds for intervention and be accompanied by a pleading that sets out the claim or defense for which intervention is sought. 
 
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