WHEN PORTLAND, Ore., lawyer Brandon Mayfield was taken into federal custody two weeks ago under a material-witness warrant, an indictment seemed likely to follow. A fingerprint belonging to Mayfield, who had converted to Islam, reportedly had been found on a bag of detonators connected to the deadly Madrid bombing, and Mr. Mayfield had done legal work for an Islamic radical successfully prosecuted in Oregon. Yet last week Mr. Mayfield walked out of detention a free man; Spanish police had reportedly tied the fingerprint on the bag to an Algerian. And the Justice Department now finds itself with some explaining to do.

A gag order on the case makes information scant, and the court, in releasing him, noted that his "release will be supervised" and that there will be "further grand jury proceedings wherein he remains a material witness." Many more facts may yet emerge. It isn't too soon, however, to ask whether someone misread a fingerprint, and if so who and why; or to worry that federal authorities might have arrested someone without the evidence to bring a responsible case.

The case is the latest example of the Justice Department's aggressive use of the power to detain "material witnesses" -- people with evidence in a court proceeding who might flee if left at large. The statute, which authorizes such detentions under court supervision for "a reasonable period of time" in order to secure testimony, is not new. It has long been used with relatively little controversy in organized-crime cases.

But since the Oklahoma City bombing investigation and particularly since Sept. 11, 2001, designation of material witnesses has become a more routine tool in the government's legal arsenal and has been deployed at an earlier stage of criminal proceedings. Rather than being used merely to ensure that witnesses are available for trial after an indictment has been issued, it has been employed to hold suspects who are themselves under investigation. The result is that dozens of people across the country have been detained for varying periods of time while the government seeks to compile evidence against them. The circumstances of these detentions are shrouded in secrecy, as are the names of the detainees and even the raw number of them. The Justice Department at times has seemed to use the statute as a kind of preventive detention law.

Congress, which has shown little interest in legislating on difficult matters involving liberty and security, has failed to clarify the new circumstances under which the material-witness law should apply. The result is a detention authority for which the parameters are dangerously undefined. How long is a "reasonable period of time" for the government to hold someone without charge while digging around to see if it can make a case? In typical criminal cases, the government is required to bring charges in order to hold any suspect. If that is to change in terrorism cases, the change should come as a result of a deliberate legislative decision, not the mission creep of an old statute envisioned for other problems.

Even if Mr. Mayfield's case does not prove to be an example of an abuse of the power, the use of the material-witness law needs careful attention and a healthy dose of sunlight.