Late Monday afternoon, facing a rebuke from the attorney general, the Immigration and Naturalization Service freed Nasser Ahmed, an Egyptian immigrant whom it had held for more than 3 1/2 years on secret evidence. As one of Ahmed's attorneys, I was elated. No one had been detained on secret evidence longer than he had. But his release still leaves another 20 or so immigration cases across the country in which the INS is using secret evidence. And the way the INS resolved Ahmed's case suggests that its main interest was avoiding a negative ruling on its broader practice.

The practice of using secret evidence is one that until recently few outside the immigrant community were even aware existed. In a criminal case, no matter how serious the charges, the government must put all its evidence on the table. Our system is based on the notion that truth is best ascertained by allowing each side to challenge the other's evidence in open court. Yet the INS claims the extraordinary power to detain and expel immigrants on the basis of evidence submitted only to an immigration judge behind closed doors. Neither the immigrant nor his attorney is entitled to see the evidence.

Several courts in the past decade have ruled that secret evidence violates the principles of due process. Yet each time the INS has lost a secret evidence case, it has sought to avoid a definitive ruling on the matter, while continuing to engage in the practice.

In 1992 a federal district court in Washington held unconstitutional the INS's use of secret evidence to expel Fouad Rafeedie, a permanent resident accused of terrorist ties to a Palestinian group. The court ruled that secret evidence "afford[s] virtually none of the procedural protections designed to minimize the risk that the government may err." The INS, which had until then argued that Rafeedie must be expelled as a threat to national security, dropped the case and allowed him to remain permanently.

In 1995 the Court of Appeals for the 9th Circuit held unconstitutional the INS's use of secret evidence to oppose two Palestinians' applications for permanent resident status. The court found that "[o]ne would be hard pressed to design a procedure more likely to result in erroneous deprivations." Again, the INS declined to appeal, and granted the aliens green cards.

Despite these defeats, the INS continued the practice. In 1998 it used secret evidence to expel a group of Iraqis who had been airlifted to this country after participating in a CIA-backed coup attempt against Saddam Hussein. When former director of central intelligence Jim Woolsey volunteered to defend the Iraqis and took their case to Congress and the press, the INS was suddenly able to declassify and disclose 95 percent of the previously secret evidence, and then settled the case by freeing the aliens and allowing them to live here in exchange for their dropping their legal challenge.

Just last month a federal district court in New Jersey ruled that the use of secret evidence to detain Hany Kiareldeen, a Palestinian, was unconstitutional. The INS had imprisoned Kiareldeen for 19 months as a national security threat, but faced with a negative court decision, it dropped its efforts to deport him and released him. Now it is seeking to have the district court decision vacated as moot.

Monday's decision in Ahmed's case is cut from the same cloth.

On Nov. 20, INS Commissioner Doris Meissner personally referred the Ahmed case to the attorney general, asking her to reverse two immigration court decisions that he should be released. Three days later, the INS filed a brief vigorously maintaining that Ahmed could not be released without endangering the nation. Yet moments before the attorney general was about to rule on the INS's referral, the INS announced it was "withdrawing" the referral. Quite plainly, the attorney general gave the INS a face-saving way to confess error.

So once again the INS freed a man who for years it had said would endanger national security by his very presence. And once again the INS avoided a definitive ruling on the use of secret evidence. As a result, we can expect that the INS will continue to engage in this practice, despite its consistent string of losses, and despite the fact that every federal court to address the practice in the past decade has declared it unconstitutional. Is this how an agency responsible for law enforcement should treat the law?

The writer, a professor at Georgetown University Law Center, has represented several immigrants in secret evidence cases.