The ruling came in the case of Abdullah al-Kidd, a star on the University of Idaho's football team.
After the Sept. 11, 2001, terror attacks, U.S. Attorney General John Ashcroft purportedly authorized law enforcement to detain terror suspects using the federal material witness law -- a potential witness who might flee can be held until needed for a prosecution.
The Justice Department said al-Kidd, a U.S. citizen who changed his name from Lavni Kidd after converting to Islam, had a close relationship with a Saudi Arabian citizen in Idaho. After the Saudi was indicted for providing money to a terrorist organization, al-Kidd attempted to board a flight to Saudi Arabia at Dulles International Airport in Virginia.
But U.S. agents had a warrant for him, issued by a magistrate who was told al-Kidd was a crucial witness in the case against the Saudi. Al-Kidd was arrested before he could leave the country and was held for more than two weeks because he refused to testify.
After his release, al-Kidd filed suit against Ashcroft's policy, alleging prosecutors never meant to call him as a witness and merely wanted to hold him as a terror suspect.
A federal judge refused Ashcroft's request to dismiss the suit on the grounds of official immunity, and a federal appeals court panel upheld the dismissal.
The appeals court said because the Fourth Amendment bans "pretextual" arrests -- arrests based on a hidden reason -- absent any evidence of criminal activity, Ashcroft and others had no immunity.
But the Supreme Court reversed. In an opinion written by Justice Antonin Scalia, the high court said, "The objectively reasonable arrest and detention of a material witness pursuant to a validly obtained warrant cannot be challenged as unconstitutional on the basis of allegations that the arresting authority had an improper motive."
The other members of the court joined Scalia's opinion or joined in the judgment, except for Justice Elena Kagan.
Kagan, who was U.S. solicitor general until joining the court this term, did not participate in the case.