The FBI's frustration over its inability to get material witnesses to talk has raised a disturbing question rarely debated in this country: When, if ever, is it justified to resort to unconventional techniques such as truth serum, moderate physical pressure and outright torture?

The constitutional answer to this question may surprise people who are not familiar with the current U.S. Supreme Court interpretation of the 5th Amendment privilege against self-incrimination: Any interrogation technique, including the use of truth serum or even torture, is not prohibited. All that is prohibited is the introduction into evidence of the fruits of such techniques in a criminal trial against the person on whom the techniques were used. But the evidence could be used against that suspect in a non-criminal case--such as a deportation hearing--or against someone else.

If a suspect is given "use immunity"--a judicial decree announcing in advance that nothing the defendant says (or its fruits) can be used against him in a criminal case--he can be compelled to answer all proper questions. The issue then becomes what sorts of pressures can constitutionally be used to implement that compulsion. We know that he can be imprisoned until he talks. But what if imprisonment is insufficient to compel him to do what he has a legal obligation to do? Can other techniques of compulsion be attempted?

Let's start with truth serum. What right would be violated if an immunized suspect who refused to comply with his legal obligation to answer questions truthfully were compelled to submit to an injection that made him do so?

Not his privilege against self-incrimination, since he has no such privilege now that he has been given immunity.

What about his right of bodily integrity? The involuntariness of the injection itself does not pose a constitutional barrier. No less a civil libertarian than Justice William J. Brennan rendered a decision that permitted an allegedly drunken driver to be involuntarily injected to remove blood for alcohol testing. Certainly there can be no constitutional distinction between an injection that removes a liquid and one that injects a liquid.

What about the nature of the substance injected? If it is relatively benign and creates no significant health risk, the only issue would be that it compels the recipient to do something he doesn't want to do. But he has a legal obligation to do precisely what the serum compels him to do: answer all questions truthfully.

What if the truth serum doesn't work? Could the judge issue a "torture warrant," authorizing the FBI to employ specified forms of non-lethal physical pressure to compel the immunized suspect to talk?

Here we run into another provision of the Constitution--the due process clause, which may include a general "shock the conscience" test. And torture in general certainly shocks the conscience of most civilized nations.

But what if it were limited to the rare "ticking bomb" case--the situation in which a captured terrorist who knows of an imminent large-scale threat refuses to disclose it?

Would torturing one guilty terrorist to prevent the deaths of a thousand innocent civilians shock the conscience of all decent people?

To prove that it would not, consider a situation in which a kidnapped child had been buried in a box with two hours of oxygen. The kidnapper refuses to disclose its location. Should we not consider torture in that situation?

All of that said, the argument for allowing torture as an approved technique, even in a narrowly specified range of cases, is very troubling.

We know from experience that law enforcement personnel who are given limited authority to torture will expand its use. The cases that have generated the current debate over torture illustrate this problem. And, concerning the arrests made following the Sept. 11 attacks, there is no reason to believe that the detainees know about specific future terrorist targets. Yet there have been calls to torture these detainees.

I have no doubt that if an actual ticking bomb situation were to arise, our law enforcement authorities would torture. The real debate is whether such torture should take place outside of our legal system or within it. The answer to this seems clear: If we are to have torture, it should be authorized by the law.

Judges should have to issue a "torture warrant" in each case. Thus we would not be winking an eye of quiet approval at torture while publicly condemning it.

Democracy requires accountability and transparency, especially when extraordinary steps are taken. Most important, it requires compliance with the rule of law. And such compliance is impossible when an extraordinary technique, such as torture, operates outside of the law.