Most terrorists camped out overseas, however, are not rushing back, and even when they are, trusting that a suspected American terrorism suspect will return home to be arrested is risky. Locals may tip him off that the United States is seeking him (yes, it is usually a him), or he may decide for his own reasons to delay or cancel his return. In addition, allowing a suspected terrorist on a plane -- even if thoroughly searched -- is nail-biting for officials involved.

So, not surprisingly, it is rare that an individual will be both known to be returning and also allowed to do so. Most of the time, officials cannot simply assume that a suspect will come back into their laps.

3. Is there a Reliable Ally to Handle the Problem?

If the suspect is abroad and not headed home, the question of which country he is in becomes important. Is he in a country whose government will reliably keep an eye on him -- and arrest him, if need be? Is that government functional enough to pull off an arrest, or does it lack control over significant parts of its territory? Is it a country that will grossly mistreat the suspect in a fashion that might prejudice his ability to get a fair trial? Is it a country with an extradition relationship with the United States?

The answers to these questions vary a great deal among the many countries in which U.S. nationals suspected of terrorism have taken refuge. If an American is living in Britain, for example, a country with strong legal institutions and a cooperative working relationship with the United States, the issue looks very different from when an American suspect is in the tribal areas of Pakistan -- or the ungoverned areas of Yemen, a country with a history of failure to prevent al Qaeda jail breaks. Americans in Somalia, a country with no functioning government at all, are a different story still. And then there's Saudi Arabia -- a highly effective police state that has had no trouble arresting American jihadists, but whose treatment of them afterward has required careful monitoring to make sure that statements given were not elicited through torture.

4. When an Ally Will Transfer

When an ally is willing to arrest and transfer a suspect to U.S. custody, suspects inevitably wind up back in criminal-justice land. This is in some ways the ideal solution, as it requires only law enforcement and intelligence cooperation with allies, not a capture operation involving U.S. forces or a kill order. But it does sometimes end up requiring delicate litigation over the suspect's treatment abroad. For example, Ahmed Omar Abu Ali -- a young man from Virginia -- was arrested in Saudi Arabia for involvement with an al Qaeda cell. He was transferred some time later to U.S. custody to face trial for conspiring to, among other things, kill President George W. Bush. The major issue in his case was whether the highly incriminating statements he made in Saudi custody were voluntary or not. Deciding this question required, remarkably, testimony by Saudi intelligence officials who interrogated Abu Ali. Abu Ali was ultimately convicted and given a lengthy prison term.