AGENTS FROM the FBI arrested Abdullah al-Kidd at Dulles International Airport in 2003. Mr. Kidd, who was headed to Saudi Arabia to study, was wanted as a material witness in an ongoing terrorism investigation.

The law allows the government to hold a material witness who is believed to be a flight risk or who is unlikely to respond to a subpoena. Mr. Kidd, a U.S. citizen whose parents and wife, also U.S. citizens, all resided in the United States, was held for 16 days in three different facilities and kept in cells that were lighted 24 hours a day; he was strip-searched multiple times. After his release, he was subjected to domestic travel restrictions for two years, forced to report his whereabouts and submit to in-home visits from a probation officer.

Mr. Kidd sued former attorney general John D. Ashcroft after the government neither charged him nor called him as a witness, arguing that Mr. Ashcroft had violated his constitutional rights by knowingly misusing the material witness warrant to detain him.

On Tuesday, a unanimous Supreme Court handed federal law enforcement a victory by ruling that Mr. Kidd was barred from suing Mr. Ashcroft. It is the right decision, but the government would be wrong to read it as a go-ahead to use material-
witness law as a pretext for preventive detention.

Law enforcement officials typically are shielded from personal liability unless they commit or authorize acts that have clearly been established to violate the Constitution. In 2003, at the time of Mr. Kidd’s arrest, no court had squarely addressed the issue of whether a material witness warrant could be used to hold an individual suspected of terrorist activity. As such, there was no way for Mr. Ashcroft to know conclusively whether such an action contravened the Constitution.

Some eight years later, the courts still have not ruled definitively on that question, and the Supreme Court didn’t address it on Monday. That leaves open the possibility that federal law enforcement officers could use the same law to detain individuals they have no intention of calling as witnesses to allow more time for law enforcement officials to dig up information on these potential suspects. In our opinion, such a practice would be wrong.

The Sept. 11, 2001, terrorist attacks encouraged law enforcement officials to tap every existing tool and to stretch their application in an effort to avert further strikes. But the government should not be in the business of contorting the material-witness statute as a means to investigate and detain suspects. The better lesson to learn from the al-Kidd case is the need for the administration and Congress to work together to establish a lawful framework with stringent judicial review to govern those rare instances when preventive detention is necessary.