THE JUSTICE DEPARTMENT started off the year with what it portrayed as a relatively clean slate in its terrorism investigation: Spokesmen announced last month that only six of the people detained in the Sept. 11 dragnet remain in U.S. custody. But that number is wildly misleading and takes advantage of a very narrow and technical definition of who counts as a detainee. In truth there are hundreds and perhaps thousands of immigrants, mostly Arabs and other Muslims, who would not be in detention but for Sept. 11, and who are now wending their way through a capricious and choked-up immigration system. Because they are not classified as "special interest" immigration cases, they receive no particular attention and aren't counted in the government's terrorism figures.

One such forgotten detainee is Ansar Mahmood, a Pakistani immigrant we wrote about on this page nearly 10 months ago. Mr. Mahmood is not counted in the Justice Department numbers, though by any reasonable definition he is a Sept. 11 detainee. He was picked up on suspicion of tainting the New York water supply and then almost immediately cleared by the FBI, which ultimately believed his story -- that he had only been photographing the scenic mountains near Rochester. He had a green card and a good job. But because police inquiries uncovered that he had helped some illegal Pakistani friends find jobs, the Immigration and Naturalization Service (INS) put him in jail pending deportation, where he has been all this time.

Mr. Mahmood decided to fight his deportation. He had the help of a lawyer and a group of citizens in Rochester who have adopted him as their cause, including his old boss, the local who had pointed him to that scenic spot and the officials at the water treatment plant who turned him in in the first place. Most of the Sept. 11 detainees don't have that kind of support, don't even have an attorney. Most of them give up and take the plane ticket home. But the hundreds of detainees immigration lawyers estimate are still in the system are likely to be cases somewhat like Mr. Mahmood's, cases compelling enough to appeal. Now their time is running out, as Attorney General John D. Ashcroft recently ordered the INS to clear out its backlog by March.

Maybe it's no big deal: one immigrant, or 100 immigrants, who ran into bad luck. All may have violated an INS regulation. But a message is sent by keeping Mr. Mahmood in jail, and by continuing more than a year later to scrutinize growing lists of Muslim immigrants -- Pakistanis, Egyptians, Kuwaitis, students. If the INS looks hard enough, it can find a technical violation by many if not most immigrants, particularly through the ever-shifting prism of the immigration bureaucracy. When those rules are enforced with exceptional zeal for a selected group, the message becomes: Terrorist or not, even legal or not, we're better off without you. And that's not true of people such as Mr. Mahmood.