The Bush administration has a problem for which it has suggested no good solution: Although hundreds of the suspected Al Qaeda members being held at Guantanamo and in Afghanistan appear to be would-be mass murderers, few seem to have been individually implicated in provable war crimes or terrorist acts. Should the Pentagon release such people-as domestic law enforcement officials would be legally obliged to do-and run the risk that they will turn to killing as many of us as they can? Or should it stretch the available evidence and the law as far as necessary to come up with some criminal charge to bring against all who seem dangerous?

It should do neither. Al Qaeda suspects who have already committed or clearly conspired to commit provable crimes should, of course, be prosecuted. But it would be a mistake to cook up new theories to bring charges against those we cannot prove guilty of conduct widely recognized as amounting to war crimes, terrorist acts, or criminal conspiracies.

The better course would be one hinted at by Defense Secretary Donald Rumsfeld but not publicly acted upon by the Pentagon. That would be indefinite detention, without criminal prosecution, of people captured on the battlefield who appear to be would-be terrorists but have not so far committed any provable crimes.

At first blush, the idea of indefinite "preventive detention" of people convicted of no crime may seem harsh and un-American. Indeed, it has (and probably should have) no basis in domestic U.S. law, with narrow exceptions such as civil commitment of the dangerously mentally ill. Prolonged detention based solely on a suspected propensity to commit future crimes may well be unconstitutional.

But in the current wartime context, these moral objections and legal obstacles are not insuperable. As a legal matter, the president has broad power to decide what to do with terrorists and others captured overseas by the U.S. military. In addition, military prisoners held abroad do not have the same rights under U.S. law as civilian prisoners in the United States. And there are international-law precedents for detaining "unlawful combatants" and members of terrorist organizations until they are no longer dangerous, as well as for detaining prisoners of war until the war is over.

As a moral matter, preventive detention is the least-bad option for dealing with many captured Al Qaeda jihadists. It’s better than setting would-be mass murderers loose to prey on our people or prosecuting them without solid evidence implicating them individually either in war crimes or in specific terrorist conspiracies.

Detainees in this category should not be criminally sentenced but rather locked up for as long as they remain dangerous. That could be many, many years. Both fundamental fairness and respect for international law and opinion would require careful, individualized proceedings to prove that each such detainee has in fact joined the jihad. We don’t want to be locking up some 19-year-old Afghan or Pakistani who once passed through an Al Qaeda camp with the sole purpose of fighting for the Taliban against the Northern Alliance.

Unfortunately, Pentagon officials are reportedly focusing on prosecuting the Guantanamo detainees, despite their apparent inability to squeeze much incriminating information out of them. Officials are considering "a new legal doctrine that would allow prisoners to be brought before military tribunals without specific evidence that they engaged in war crimes," Neil A. Lewis reports in The New York Times; for example, senior Al Qaeda members could be held responsible for war crimes committed by other members of the same units.

An even broader theory of prosecution could invoke the long-established law of criminal conspiracy against anyone who knowingly signed on to Al Qaeda’s global terrorist jihad. Under federal law, a person who agrees to collaborate in a succession of murders can theoretically be convicted of murder conspiracy even if he or she has as yet done little or nothing to carry out the conspiracy and is unaware of what other co-conspirators are doing. So any Al Qaeda member could theoretically be charged with conspiring to murder Americans and others. And the same evidence that shows dangerousness would go at least part of the way toward proving conspiracy.

But such a prosecution could present formidable problems of proof. The only evidence against many of the detainees may be that they were non-Afghans fighting in Al Qaeda or Taliban units that committed war crimes in Afghanistan, or that they trained in Al Qaeda camps, as evidenced by captured documents. Criminal prosecutions based solely on evidence as thin as that would stretch the law of conspiracy and its military analogues, including "command responsibility," to unnecessary extremes.

Al Qaeda is a vast, worldwide collection of loosely affiliated groups. Killing Americans is not the only thing that it does. And jihadists do not carry membership cards. So a defendant’s membership in Al Qaeda might be hard to establish. And proof of membership alone might not be very convincing evidence that the individual member had conspired to kill Americans or to commit any traditionally recognized war crime.

This could explain why the administration did not charge Taliban member John Walker Lindh, an American, with conspiring to kill Americans in the United States on September 11, or ever: Although he had trained at Al Qaeda camps, and had met with Osama bin Laden, he turned down an opportunity to undergo intensive training for terrorist attacks overseas, choosing instead to fight on the Taliban front lines. Whether he had become a member of Al Qaeda seems debatable. (The charges against Lindh do include conspiracy to murder Americans in Afghanistan.)

Prosecuting hundreds of Arabs and Muslims before military commissions, without proof beyond a reasonable doubt of personal involvement in recognized war crimes or terrorist acts, might result in many acquittals. And any convictions might not be persuasive enough to rebut the inevitable chorus of "kangaroo-court" complaints in Europe-where we need all the help we can get in our war against Al Qaeda-and elsewhere.

It’s also possible (if unlikely) that federal courts would seek to overturn convictions of doubtful merit, especially if Guantanamo fills up and some such prisoners are moved to the United States. And therein lurks a risk of dangerous confrontation between the courts and the commander in chief.

We need not strain so hard to convict every suspected Al Qaeda jihadist of some crime. Instead, the administration should align its legal grounds and process for locking up so-far-inactive would-be terrorists with the real reason for locking them up. The real reason is not to punish them for past crimes. It is to prevent future crimes. And the same evidence of knowing association with Al Qaeda’s terrorist agenda that could provide a weak basis for criminal prosecution would provide a solid basis for preventive detention.

Here’s how such a process could work: President Bush or the Pentagon could assign the special military commissions that Bush authorized in November to not only try criminal charges but also, in appropriate cases, order indefinite detentions of dangerous people captured on the battlefield. Such detention orders could be based upon clear and convincing evidence that the detainees pose serious threats to the lives of Americans or others, and could continue for as long as the detainee remains dangerous. Given the fanatical thirst of many detainees for American blood, that might end up being for as long as Al Qaeda continues to wage war against us. Or longer.

A preventive detention regime along these lines should pass muster under a reasonable interpretation of either of two international-law doctrines. One has been detailed in decisions of the European Court of Human Rights upholding Britain’s right to detain (but not to mistreat) Irish Republican Army members suspected of terrorist plans. The other is the long-established rule approving detention of prisoners of war and unlawful combatants.

To be sure, the prisoner-of-war rules have been applied to nation-states, not terrorist groups, and require that POWs be repatriated promptly once the war is over (because they no longer pose any danger). But Al Qaeda has more capacity than most nation-states to inflict carnage on the United States and its people. And while the war in Afghanistan may soon be over, the war against Al Qaeda will not. The world has changed. International law should adapt.

Preventive detention would also avoid the sticky problem of deciding what sentences to impose. Convictions based on weak evidence, or for relatively minor crimes, would not provide compelling justifications for long prison terms (let alone death sentences). So absent preventive detention, we might find ourselves letting would-be mass murderers loose after a few months or years. Why would we want to do that?