Abdullah al-Kidd, a U.S. citizen, is a onetime University of Idaho football star, born Lavoni T. Kidd. He converted to Islam in college. He was arrested at Dulles International Airport in 2003 as he was boarding a plane for Saudi Arabia, where he planned to study.

The government persuaded a federal judge to issue a warrant for Kidd's arrest by saying he was necessary to the investigation of Sami Omar al-Hussayen, who was eventually indicted on charges of supporting terrorism. Kidd was never called to testify against Hussayen, who was acquitted of the most serious charges against him.

Kidd maintains that in his more than two weeks of detention, he was strip-searched, shackled, interrogated without an attorney present and treated as a terrorist.

Even though Wednesday's argument was limited to the question of whether Ashcroft had immunity, Justice Ruth Bader Ginsburg took note of Kidd's allegations.

"There are allegations here that this man was kept awake, the lights shining in his cell for 24 hours, kept without clothes," Ginsburg said. "Now that doesn't sound like the way one would treat someone whose testimony you want."