The Immigration Act of 1965 generated many unanticipated crises in imposing immigration restrictions on U.S. neighbors within the western hemisphere. Previously, immigration had not been quantitatively limited and subject to selective criteria with many hemispheric travelers handled as “nonimmigrants.” After 1965, however, more intensified enforcement efforts, increasing immigration, and more immigration from countries further south produced growing problems of unauthorized entries and efforts at settlement. Large influxes of asylum seekers from Guatemala, El Salvador, Nicaragua, and Honduras during the 1980s, who were largely categorized as “economic refugees” and refused permanent status by the Reagan administration. Many nonetheless remained in the United States with the support of widespread humanitarian protesters who laid the foundations of the modern sanctuary movement and court challenges for the rights of those seeking asylum. Zadvydas v. Davis was a key ruling that provided some civil rights protections and limited the powers of immigration authorities to indefinitely detain persons denied asylum.
JUSTICE BREYER delivered the opinion of the Court. When an alien has been found to be unlawfully present in the United States and a final order of removal has been entered, the Government ordinarily secures the alien’s removal during a subsequent 90-day statutory “removal period,” during which time the alien normally is held in custody.
A special statute authorizes further detention if the Government fails to remove the alien during those 90 days. It says:
“An alien ordered removed  who is inadmissible …  [or] removable [as a result of violations of status requirements or entry conditions, violations of criminal law, or reasons of security or foreign policy] or  who has been determined by the Attorney General to be a risk to the community or unlikely to comply with the order of removal, may be detained beyond the removal period and, if released, shall be subject to [certain] terms of supervision .. “8 U. S. C. § 1231(a)(6) (1994 ed., Supp. V).
In these cases, we must decide whether this postremoval-period statute authorizes the Attorney General to detain a removable alien indefinitely beyond the removal period or only for a period reasonably necessary to secure the alien’s removal. We deal here with aliens who were admitted to the United States but subsequently ordered removed. Aliens who have not yet gained initial admission to this country would present a very different question. See infra, at 693-694. Based on our conclusion that indefinite detention of aliens in the former category would raise serious constitutional concerns, we construe the statute to contain an implicit “reasonable time” limitation, the application of which is subject to federal-court review.