It does not resolve the question of whether the Fourth Amendment permits the arrest of an individual—using the material witness statute as a subterfuge—without probable cause to believe that he has committed a crime. It does not resolve the question of whether the absolute and qualified immunity standards are the same for an Assistant United States Attorney as the Attorney General and all the supervisory attorneys in between. He then sued a line of federal officials from the FBI to the Attorney General, claiming a violation of the Fourth and Fifth Amendments and of the material witness statute: 
Al-Kidd asserts three independent claims against Ashcroft. The defendants moved to dismiss for failure to state a claim and on grounds of absolute and qualified immunity. Prosecutors enjoy absolute immunity from suit for what they do in court. They enjoy qualified immunity for work they do as investigators. The federal material witness statute provides that
If it appears from an affidavit filed by a party that the testimony of a person is material in a criminal proceeding, and if it is shown that it may become impracticable to secure the presence of the person by subpoena, a judicial officer may order the arrest of the person and treat the person in accordance with the provisions of section 3142 of this title [relating to bail]. 18 U.S.C. An arrest warrant for a witness with evidence material to a federal criminal proceeding may be issued by federal or state judges or magistrates. Absolute Immunity

Prosecutorial officials enjoy absolute immunity from civil suit for their actions as officers of the court. Qualified Immunity

"Most public officials are entitled ... to qualified immunity" for acts committed in the performance of their duties. "The doctrine of qualified immunity protects government officials 'from liability for civil damages insofar as their conduct does not violate clearly established statutory or constitutional rights...." 
Al-Kidd claimed that his Fourth Amendment rights had been violated by policy created and implemented by the Attorney General used the material witness statute as a pretext to arrest criminal suspects. The material witness statute requires probable cause to believe that an individual has evidence material in a judicial proceeding. The Fourth Amendment requires probable cause to believe that an individual has committed a crime. The two are not the same. To use a material witness statute pretextually, in order to investigate or preemptively detain suspects without probable cause is to violate the Fourth Amendment." The Ninth Circuit, however, accepted al-Kidd's contention that the ordinary rules did not apply because use of the material witness statute did not deal with the ordinary type of probable cause upon which the Supreme Court's cases were predicated: probable cause to believe a crime had been committed. (2) Ashcroft was entitled to qualified immunity, because arrest under a valid material witness arrest warrant was not a violation of the Fourth Amendment regardless of the prosecutor's motives. Whether for want of a Fourth Amendment violation or want of a clearly established Fourth Amendment violation, all of the Justices agreed that the Attorney General was entitled to qualified immunity.
Public officials cannot be sued personally for injuries resulting from the performance of their duties. They lose this qualified immunity when the injuries resulted from their violation of clearly established law. In Ashcroft v. al-Kidd, the Supreme Court concluded that clearly established Fourth Amendment jurisprudence did not forbid the Attorney General from encouraging the use of the federal material witness statute as a pretext to detain and question a potential criminal suspect.
The Court left for another day several related issues. Does the material witness statute permit authorities to arrest, question, and detain an individual because he is criminal suspect rather than a witness? Is the material witness statute unconstitutional on its face? Does the Fourth Amendment permit recourse to the material witness statute in order to arrest, question, and detain an individual without probable cause to believe he has committed a crime? Does the absolute immunity that attends the performance of duties in a judicial environment apply? Are the absolute and qualified immunity standards the same for local prosecutors as for the Attorney General?
Al-Kidd was arrested on a federal material witness warrant. Federal law permits the arrest of a witness when there is probable cause to believe that he has evidence material in a judicial proceeding and may be unavailable when his testimony is needed. Arrest of a criminal suspect requires probable cause to believe that he has committed a crime. Al-Kidd sued the Attorney General alleging that, in the wake of the 9/11 attacks, the Justice Department had created and implemented a policy of arresting terrorist suspects without criminal probable cause by using the material witness statute as a subterfuge. The Attorney General moved to dismiss on grounds of absolute and qualified official immunity. Prosecutors are entitled to absolute immunity from suit for activities performed as officers of the court. They are entitled to no more than qualified immunity for activities performed as investigators. They are entitled to qualified immunity for the performance of their official duties, unless they violate a clearly established constitutional or statutory right.
The Ninth Circuit Court of Appeals held that (1) prosecutors who use material witness warrants as an investigative tool are not entitled to absolute immunity; (2) prosecutors, who use such warrants without probable cause to believe the witness has committed a crime, violate the clearly established Fourth Amendment proscription on unreasonable searches and seizures; (3) prosecutors who do so are not entitled to qualified immunity; and (4) the same is true of senior officials who direct such misconduct.
Justice Scalia, writing for the Court, resolved the matter narrowly. He observed that in most instances, the Fourth Amendment reasonableness of an arrest is judged objectively. An otherwise reasonable arrest is ordinarily permissible regardless of the alleged motives of the arresting officers. Where there is probable cause for the arrest of a material witness, motive is irrelevant, and the arrest is reasonable by Fourth Amendment standards. Since the Attorney General thus did not instigate a clearly established violation of the Fourth Amendment as alleged, he was entitled to qualified immunity from suit.
Related reports include CRS Report R41903, Federal Material Witness Statute: A Legal Overview of 18 U.S.C. 3144, by [author name scrubbed].