On May 31, 2011, the Supreme Court decided Ashcroft v. al-Kidd, No. 10-98, holding that the objectively reasonable arrest and detention of a material witness under the federal material-witness statute (18 U.S.C. § 3144) and a validly-obtained warrant cannot be challenged as unconstitutional based on allegations that the arresting authority had an improper motive in making the arrest.

In the aftermath of the September 11 terrorist attacks, then-Attorney General John Ashcroft authorized federal prosecutors and law-enforcement officials to use the federal material-witness statute, 18 U.S.C. § 3144, to detain people with suspected ties to terrorist organizations. The statute authorizes federal 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 present of the person by subpoena." The statute requires the release of the witness if his testimony "can adequately be secured by deposition" and "further detention is not necessary to prevent a failure of justice."

FBI agents obtained a warrant under 18 U.S.C. § 3144 to arrest and detain Abdullah al-Kidd, a U.S. citizen, on the ground that he had information that was crucial to the prosecution of Sami Omar al-Hussayen. Agents arrested al-Kidd as he checked in for a flight to Saudi Arabia. He remained in custody for 16 days, and on supervised release until al-Hussayen's trial ended 14 months later. The government never called al-Kidd as a witness.

al-Kidd sued Ashcroft and others in a Bivens action, arguing that the government never intended to use him as a witness at al-Hussayen's trial, and that his arrest and detention were simply a pretext. Ashcroft moved to dismiss the case based on absolute and qualified immunity. The District Court denied the motion, and the Ninth Circuit affirmed the denial, holding that pretextual arrests without probably cause of criminal wrongdoing clearly violate the Fourth Amendment, so immunity was not available.

Qualified immunity shields government officials from lawsuits seeking money damages unless the official 1) violated a statutory or constitutional right, and 2) the right was "clearly established" at the time of the challenged conduct. The Court observed that while courts should be cautious in resolving difficult and novel questions of constitutional or statutory interpretation when it is not necessary, the Court would exercise its discretion to take up the issue whether the officials violated a statutory or constitutional right in the first place, because the Ninth Circuit had decided both of the qualified-immunity inquiries and its analysis of the constitutional issue "needs correction."

The Court held, based on its prior Fourth Amendment cases, that the question of reasonableness under the Fourth Amendment is an objective inquiry, and that the question is whether the circumstances, viewed objectively, justified the official's actions regardless of the official's subjective intent in taking those actions. In this case, the existence of a judicial warrant based on individualized suspicion gave the officials an objective, reasonable basis to take the action that they did. Thus, the Court held, there was no violation of the Fourth Amendment. And there was also no "clearly-established" law that the officials violated. Thus, they were entitled to immunity from suit.

Justice Scalia delivered the opinion of the Court, in which Chief Justice Roberts and Justices Kennedy, Thomas, and Alito joined. Justice Kennedy filed a concurring opinion, Part I of which Justices Ginsburg, Breyer, and Sotomayor joined. Justice Ginsburg filed an opinion concurring in the judgment, in which Justices Breyer and Sotomayor joined. Justice Sotomayor filed an opinion concurring in the judgment, in which Justices Ginsburg and Breyer joined. Justice Kagan did not participate.

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