Federal statute 18 U.S.C. § 3144 authorizes a magistrate to issue a warrant to arrest someone as a material witness based on an affidavit showing that the testimony of the person is material in a criminal proceeding and that it would be impracticable to use a subpoena to secure his presence. In 2003, Abdullah al-Kidd (plaintiff), an American citizen, was at the airport on his way to study in Saudi Arabia on scholarship when he was arrested pursuant to a material-witness warrant (MWW). The warrant had been issued based on an affidavit from the Federal Bureau of Investigation (FBI), asserting that al-Kidd was suspected of having information linked to the prosecution of Sami Omar Al-Hussayen for various fraud-related charges. Al-Kidd was handcuffed, interrogated, and detained for over two weeks, during which time he claims he was treated harshly. Al-Kidd was never called as a witness, and there was no evidence he committed any crime himself. Al-Kidd brought suit against Attorney General Ashcroft and others (defendants) for injuries resulting from the claimed misuse of material-witness detention and the conditions of his confinement. The defendants moved to dismiss the case, claiming qualified immunity, and the district court denied the motion. The defendants then appealed, and the United States Court of Appeals for the Ninth Circuit affirmed. A rehearing en banc was denied, and the defendants petitioned for certiorari. The Supreme Court granted their petition.