FEW ASPECTS of the government's domestic response to the Sept. 11, 2001, attacks have been more shrouded in secrecy than its use of a relatively obscure federal law that permits the detention of "material witnesses" in federal criminal proceedings. The government has kept mum on the subject, citing grand jury secrecy and court orders in declining to disclose the names of those it has locked up, the lengths of their detentions, or even the total number of detainees. How the Justice Department has used the law is, therefore, something of a mystery. But the few windows the public has on the matter raise questions. The law was intended to ensure that the government could secure people's testimony, and to that end it permits the department, under court supervision, to hold for a "reasonable period of time" those witnesses who might otherwise flee. The question is whether the government is stretching this purpose and using the law as a preventive detention regime for those whom it regards as dangerous yet lacks evidence to prosecute.

There is no question the Justice Department is using the law far more aggressively than it has in the past. A recent story by Post staff writers Steve Fainaru and Margot Williams reports that of 44 known material witnesses, at least 20 never testified before a grand jury. Though most of these people have been released, some were held for long periods of time. This does not necessarily imply anything untoward. After all, it is possible that a witness might legitimately be held for a few months for repeated grand jury appearances; how long is "reasonable" depends on the circumstances, and the circumstances following the attacks could have led sensible judges to give the Justice Department latitude. Moreover, it is also possible that prosecutors could have lost interest in certain people arrested as material witnesses and released them without bothering to get their testimony. But the number of people released without testifying raises the question of whether securing testimony was the purpose in some of these detentions -- as opposed to holding people while their backgrounds were scoured.

Department officials emphasize that all of the material witness detentions are supervised by judges and that attorneys are appointed for the detainees. And while there is no fixed limit on how long someone can be held, they say, the purpose must be to secure testimony -- either before a grand jury or in less formal settings -- not to keep dangerous people locked up. But with the government so tight-lipped about the specific cases, it is hard to find these affirmations reassuring. The department's secrecy, moreover, is itself troubling; a federal judge has actually rejected blanket secrecy in this area -- only to have the department appeal and argue in favor of continued gagging. Until the gag comes off, it is impossible to know whether this law is being used aggressively yet responsibly or whether it is being twisted toward a purpose Congress never intended.