IT'S HARD to imagine a better example of the dangers of secret evidence than the case of Nasser Ahmed. Mr. Ahmed, an Egyptian who worked as a translator for Sheik Omar Abdel Rahman during his trial, has been held for more than three years, mostly in solitary confinement, as the government has sought to deport him. At first the INS would give him no information about why he was regarded as a security risk. An immigration judge ruled in 1997 that if he were returned to Egypt, he would likely be tortured because of his political associations and that he was, on the basis of the public record, eligible for asylum here. But the judge denied his asylum request on the strength of the government's secret evidence.

Turned out, however, that the national security interest in keeping this material under wraps was not all that compelling after all. The INS, in response to a subsequent lawsuit, released summaries of the material and declassified some of the material itself, and Mr. Ahmed, at long last, had a chance to attack it. The allegations included involvement in the sheik's terrorist group and carrying a letter from the imprisoned cleric to the press that was somehow linked to a subsequent terrorist attack.

Yet once the proceeding became more genuinely adversarial, Mr. Ahmed's case began to look rather different. And last week, the same immigration judge who had once declined to stop his deportation reversed himself, granted asylum and ordered Mr. Ahmed released. "Armed with a better understanding of the government's case, [Mr. Ahmed] was successful in rebutting most of the factual allegations underlying the charge that he is a danger to the security of the United States," the judge wrote. Even the still-secret evidence "can no longer be viewed as sufficiently reliable to support a finding that [Mr. Ahmed] is a danger." The government is appealing, and Mr. Ahmed remains in detention while the appeal is pending. (He has also, in a separate matter, been convicted of making false statements on an immigration form.)

The use of secret evidence is antithetical to the premises of our judicial system. While it is possible to imagine immigration cases in which secrecy might be necessary, the government has undermined confidence that it is making responsible judgments about such occasions. In this case, it released information that it had argued could not safely be released; when that information was subjected to scrutiny, the government's case melted away. It will be difficult to trust the next claim that a given person is so dangerous that he must be deported -- and the reasons so secret they cannot be disclosed.