533US2 Unit: $U87 [10-21-02 18:35:10] PAGES PGT: OPIN Cite as: 533 U. S. 678 (2001) 699 Opinion of the Court We have found nothing in the history of these statutes that clearly demonstrates a congressional intent to authorize indefinite, perhaps permanent, detention. Consequently, interpreting the statute to avoid a serious constitutional threat, we conclude that, once removal is no longer reasonably foreseeable, continued detention is no longer authorized by statute. See 1 E. Coke, Institutes *70b (“Cessante ratione legis cessat ipse lex”) (the rationale of a legal rule no longer being applicable, that rule itself no longer applies). IV The Government seems to argue that, even under our interpretation of the statute, a federal habeas court would have to accept the Government’s view about whether the implicit statutory limitation is satisfied in a particular case, conducting little or no independent review of the matter. In our view, that is not so. Whether a set of particular circumstances amounts to detention within, or beyond, a period reasonably necessary to secure removal is determinative of whether the detention is, or is not, pursuant to statutory authority. The basic federal habeas corpus statute grants the federal courts authority to answer that question. See 28 U. S. C. § 2241(c)(3) (granting courts authority to determine whether detention is “in violation of the . . . laws . . . of the United States”). In doing so the courts carry out what this Court has described as the “historic purpose of the writ,” namely, “to relieve detention by executive authorities without judicial trial.” Brown v. Allen, 344 U. S. 443, 533 (1953) (Jackson, J., concurring in result). In answering that basic question, the habeas court must ask whether the detention in question exceeds a period reasonably necessary to secure removal. It should measure reasonableness primarily in terms of the statute’s basic purpose, namely, assuring the alien’s presence at the moment of removal. Thus, if removal is not reasonably foreseeable, the court should hold continued detention unreasonable and no