Last week, the government announced it would prosecute alleged 9/11 mastermind Khalid Sheikh Mohammed and four coconspirators in federal court. Given the government's acknowledgment that Mohammed and his codefendants were subject to harsh interrogation techniques, including repeated waterboarding, the decision puts the issue of torture squarely before the federal courts.

To understand how the government cannot avoid the issue, one needs to know something about a rare and somewhat forgotten defense in criminal law known as "outrageous government misconduct."

As a former federal prosecutor and a participant in some of the Department of Justice's conventional investigative efforts in the days after 9/11, I am sure the government's case will be overwhelming. I doubt that Mohammed's viable defenses include that he was not a conspirator in the 9/11 attacks.

Nor is Mohammed likely to accomplish much by arguing that the government's evidence should be excluded because it's tainted by his treatment while in U.S. custody. The prosecutors would never have pursued this case unless they were confident they could win it solely with evidence gathered before Mohammed's capture. Mohammed's treatment at Guantanamo and elsewhere gives him no legal basis to suppress evidence gathered before he was in custody.

The prosecutors' challenge will be to avoid outright dismissal of the case before trial, when Mohammed is sure to assert the "outrageous government misconduct" defense.

This defense is based on the theory that, regardless of evidence of guilt, the Constitution prohibits prosecution of a person in a case where government agents engaged in behavior that "shocks the conscience."

The Supreme Court has not considered this defense for a long time, but it hasn't rejected it, either. The defense stems from a 1952 decision authored by Justice Felix Frankfurter. In that case, the court ruled that California violated the Constitution when it arrested a drug suspect, took him to a hospital, directed a doctor to pump his stomach, and then seized drugs the man vomited as evidence to prosecute him.

This was not ruled an illegal search under the Fourth Amendment. But it was outrageous conduct that violated the guarantee of due process, Frankfurter wrote; the government's methods were "too close to the rack and the screw" for the Constitution to tolerate. Prosecution dismissed.

In the years since this decision, the federal appellate courts have been divided over the continued viability of the outrageous-misconduct defense. But several have squarely said that it exists. They include the influential U.S. Circuit Court of Appeals for the District of Columbia, which has said that such an outright dismissal of a case should be reserved for instances of "coercion, violence, or brutality to the person."

The applicability of the defense to the prosecution of Mohammed is obvious, given his repeated waterboarding, among other official acts. Because the government used methods that were so "close to the rack and screw," it seems that a court could avoid dismissing the prosecution only by eliminating the outrageous-misconduct defense - that is, ruling that it no longer exists.

The Supreme Court has never said this, and the appeals courts are divided on the question. Meanwhile, the present Supreme Court has been anything but reluctant to address major legal questions flowing from the government's war with al-Qaeda. As such, Mohammed's case would seem bound for the Supreme Court, maybe even before he can be tried.

On its way there, it will force the federal courts to confront the central questions in the debate over torture. The unavoidable issues include whether what was done to Mohammed "shocks the conscience," because it was indeed, by anyone's sensible definition, torture. The courts will also have to decide whether the government is so culpable in the victimization of the defendant that it has forfeited its right to call him to account for his role in the murders of the victims of 9/11.