The federal material-witness statute authorizes judges to “order the arrest of [a] person” whose testimony “is material in a criminal proceeding . . . if it is shown that it may become impracticable to secure the presence of the person by subpoena.” 18 U.S.C. § 3144. P filed a Bivens suit that alleges that, in the aftermath of the September 11th terrorist attacks, D authorized federal prosecutors and law enforcement officials to use the material-witness statute to detain individuals with suspected ties to terrorist organizations. P alleges that D had no intention of calling most of these individuals as witnesses and that they were detained, at D's direction, because federal officials suspected them of supporting terrorism but lacked sufficient evidence to charge them with a crime. Two days earlier, they had informed a Magistrate Judge that, if P boarded his flight, they believed information “crucial” to the prosecution of Sami Omar al-Hussayen would be lost. P remained in federal custody for 16 days and on supervised release until al-Hussayen's trial concluded 14 months later. P was never called as a witness. D filed a motion to dismiss based on absolute and qualified immunity. The District Court denied the motion. The Ninth Circuit affirmed, holding that the Fourth Amendment prohibits pretextual arrests absent probable cause of criminal wrongdoing and that D could not claim qualified or absolute immunity. The Supreme Court granted certiorari.