The Supreme Court may answer that question in Ashcroft v. Al-Kidd, a case the Court announced this morning that it will hear later in the Term. If they answer it, my guess is that they’ll use the standard that lower courts have unanimously approved in the civil commitment setting. In civil commitment cases, the question is what standard the Fourth Amendment requires to detain someone because he is dangerous to others or himself. Lower courts have held that probable cause to believe the person poses a danger to himself or others satisfies the Fourth Amendment. Here’s how one circuit court analyzed the issue in Pino v. Higgs, 75 F.3d 1461 (10th Cir. 1996):
The Fourth Amendment is not limited to criminal cases, but applies whenever the government takes a person into custody against her will. In re Barnard, 455 F.2d 1370, 1373-74 (D.C.Cir.). A seizure does not violate the Fourth Amendment, however, unless it is also unreasonable.
In the criminal arrest context, a Fourth Amendment seizure is reasonable if it is based on “probable cause.” Because similar underlying interests arise in the context of a detention for an emergency health evaluation, several courts have applied an analogous “probable cause” doctrine in determining the validity of the government’s seizure of a person for mental health reasons. See, e.g., Villanova v. Abrams, 972 F.2d 792, 795 (7th Cir.); Gooden v. Howard County, 954 F.2d 960, 968 (4th Cir.) (en banc); Harris v. Pirch, 677 F.2d 681, 686 (8th Cir.); Baltz v. Shelley, 661 F.Supp. 169, 178-79 (N.D.Ill.).
The state has a legitimate interest in protecting the community from the mentally ill and in protecting a mentally ill person from self-harm. A person suspected of mental illness possesses a right to liberty and a right to freedom from unfounded charges of mental infirmity. Because a seizure of a person for an emergency mental health evaluation raises concerns that are closely analogous to those implicated by a criminal arrest, and both are equally intrusive, we conclude that the “probable cause” standard applies here . . . .
In effect, the difference between the criminal arrest and the civil commitment detention becomes what kind of probable cause is required: While the criminal arrest requires probable cause that the person committed a crime, civil detention requires probable cause that the person poses a danger to himself or others.
The Supreme Court has never addressed the issue, but I wouldn’t be surprised if the Justices adopted the civil commitment standard in the case of national security detention. After all, the point of national security detention is quite similar to the point of civil commitment: The goal is to stop and monitor someone who is believed to be a threat of engaging in violent acts. If probable cause to believe the person is dangerous permits a detention for civil commitment purposes in the mental health setting, I would not be surprised if the Justices applied the same reasoning to say that the same standard allows allows detention in national security cases.
UPDATE: I should make clear that the probable cause standard permits detention but not indefinite detention; similarly, I am referring to temporary detention for national security purposes rather than some indefinite detention for such purposes. My point was simply to note that the civil commitment cases present a body of lower-court caselaw that circuit courts have followed for a long time that a majority of the Justices might find a helpful starting point for thinking about national security detention.