THE FBI has graciously, and properly, apologized to Oregon lawyer Brandon Mayfield. Mr. Mayfield, a convert to Islam, was detained for two weeks as a material witness in connection with the March 11 Madrid bombings. He was released and exonerated when the FBI conceded that a fingerprint analysis linking him to a bag of detonators found by Spanish authorities was wrong.

In complex investigations, mistakes happen, and in this case not only the FBI erred. Court records released this week indicate that a court-appointed fingerprint expert paid for by the defense concurred with the bureau's judgment that Mr. Mayfield's prints matched the one on the bag. It also is clear that the FBI did not initially take seriously enough the reservations of Spanish police about its analysis. It has announced that it is reexamining its procedures -- as it should.

But neither the bureau's apology nor its self-examination speak to the fundamental issue of how the government is using the material-witness law. Mr. Mayfield was never charged with a crime, nor would any reasonable prosecutor have brought charges based on what the government knew -- or thought it knew -- about him. Rather, like many people since Sept. 11, 2001, he was held under a law that permits the detention, under court supervision, of people with evidence relevant to a court proceeding or grand jury inquiry who might flee if left at large. The law is not new, but its aggressive use at the investigative stage of high-profile cases is. In some instances, the Justice Department's use of it has smacked of preventive detention. The secrecy surrounding these cases makes a thorough assessment impossible. In Mr. Mayfield's case, the government's use of the law allowed the detention and public smearing of a man against whom the government was not prepared to make a case and whom it now concedes to be innocent.

Unfortunately, the one institution that could do a thorough assessment hasn't done so. Congress has done little to study the use of this law in the war on terrorism, or to consider whether the authority to detain material witnesses needs tailoring. Members have written letters seeking information from the Justice Department, and the matter has arisen at several hearings. But sustained legislative interest has been sadly lacking. That has to change; the bureau briefed congressional staff members Tuesday, and Senate Judiciary Committee Chairman Orrin G. Hatch (R-Utah) means to seek a written explanation of the failure, his office says. This is a start. Congress should not continue to ignore its responsibility to examine how this law has evolved and whether its capacity to infringe on the liberty of innocent people needs to be checked.