Shortly after the September 11 terrorist attacks, then-Attorney General John Ashcroft announced a “paradigm of prevention,” by which the Justice Department would aggressively use federal law to take “suspected terrorists” off the streets. As he later admitted in his book Never Again, however, Ashcroft and those he oversaw had no idea where the next terrorist threat might lie. So the government locked up more than 5,000 foreign nationals in anti-terrorism preventive detention in the first two years after 9/11–not one of whom stands convicted of a terrorist offense today.

Most were locked up using immigration law. But when the government suspected a US citizen, immigration law was no help. So the Ashcroft Justice Department turned to the “material witness” law, which applies to citizens, and twisted it for ends it was never designed to serve–to lock up people “for investigation” even where the government lacked probable cause to believe they had engaged in criminal activity.

Abdullah al-Kidd was one of the many locked up under this law. A US citizen born in Wichita and once a star running back for the University of Idaho, al-Kidd was arrested as a material witness in 2003 and spent more than a year living under strict, parole-like conditions. He was never called to testify in any proceeding. Like many others, he was never charged with criminal conduct. He was a victim of Ashcroft’s “paradigm of prevention.”

Al-Kidd sued, and in September the US Court of Appeals for the Ninth Circuit in San Francisco ruled that Ashcroft could be held personally responsible for this policy. This is a landmark decision for two reasons: it declares a central aspect of Ashcroft’s policy clearly illegal, and it insists that accountability extends to those who authorize illegal policies, not just to the foot soldiers who carry them out.

Al-Kidd was subjected to an FBI investigation after 9/11; he was, after all, a Muslim. But the FBI found no evidence that he had engaged in criminal activity. That should have been the end of the matter. But when al-Kidd decided to go to Saudi Arabia in 2003 to study Arabic and Islamic law, he again aroused the government’s suspicions.

So it turned to the material witness law, which allows for the imprisonment of a witness when it can be established that the witness’s testimony is relevant to a criminal investigation or trial and that the witness is likely to flee if subpoenaed. Properly used, the law has a legitimate purpose; but it is vulnerable to being abused as a ruse for detaining people for whom the government lacks probable cause of criminal activity. Under Ashcroft, that abuse became official Justice Department policy. Al-Kidd was arrested purportedly in connection with the criminal trial of Sami al-Hussayen, a University of Idaho student prosecuted by the government for allegedly providing material support to terrorists by running a website with links to other websites featuring jihadist rhetoric. Al-Hussayen was acquitted of those charges. Al-Kidd was never called to testify in that trial or any other.

When al-Kidd sued Ashcroft, Ashcroft argued that the case should be dismissed at the outset. He maintained that he’d played no personal role in al-Kidd’s detention; that the law against using material-witness authority for investigative purposes was not sufficiently clear; and that he should be immune under a doctrine that guards prosecutors from civil suits for their prosecutorial decisions.

The court of appeals, in a decision written by a George W. Bush nominee, rejected all of these arguments. It declared that Ashcroft could be held responsible for injuries inflicted pursuant to his stated policy of using the material witness law to round up suspected terrorists. And it ruled that exploiting the law to lock up suspects for investigative or preventive purposes when probable cause of criminal activity is lacking is clearly unconstitutional under the Fourth Amendment.

The decision is significant not only for al-Kidd but for all who might be deemed “suspicious” after a future terrorist attack. It puts government officials on notice that “preventive” measures must respect fundamental constitutional guarantees, and that when they do not, accountability will go to the top. Those are lessons that could profitably be applied more broadly, especially to those who authorized torture, as we begin to account for the wrongs committed in the name of the “war on terror.”