Ahmed found it odd that he was constructing a giant vat for production of specialized proteins in an unmarked complex of buildings far off the main road south of Baghdad. But he knew enough not to ask too many questions.

In long years of service to the military-industrial ministries of President Saddam Hussein's government, Ahmed had learned not to inquire about the ultimate uses of the projects he worked on, first as a nuclear construction engineer during the 1980s and then on this seemingly innocuous pot.

"This was a regime that got used to hiding things. We didn't need to know, until it became obvious what it was about," he said.

In the case of the vat, Ahmed had his suspicions. He thought it was meant to create biological weapons material. He was apparently right. U.N. inspectors dismantled the equipment in the late 1990s, after a high-level defector tipped them off to its uses. "That's what the inspectors looking around Iraq now will need," he said, "a defector."

The Bush administration is pressing the current U.N. inspection team to ferry scientists out of Iraq for interrogation. Only then, administration officials say, will they get useful information on suspected Iraqi nuclear, chemical and biological arms programs. Failure of Hussein to permit scientists and their families to leave would, in the administration's view, constitute a breach of the latest U.N. resolution demanding open access to weapons sites.

Ahmed left Iraq in 1999 and lives in an Arab country. On a visit to London, he discussed his experiences in fooling earlier weapons inspectors, but asked to keep his real name out of print, because of fears for relatives still living in the country -- one reason he is an example of why the Bush administration says an interrogations-abroad program is necessary and why it might not work.

"Even if you take out their wives and kids, they have other relatives in Iraq -- brothers, cousins, mothers, fathers. Saddam can have them all killed," he said. "You would have to be able to provide the scientist and everyone else full security. They would have to believe that Saddam could not get his hands on them.

"Also, the scientists may not have anything to say. There is no new science in Iraq. The programs, if any, are in the hands of security people. Take me. I could say what I worked on, but I could not tell you the state of any program that went on after I stopped working. Only a few people have that kind of information, and they are well hidden."

Ahmed said he believes that the Iraqi government is continuing to develop biological and chemical weapons and also has become more adept at hiding the programs. "They have had lots of practice," he said.

Ahmed is no repentant defector. He proudly recounted his career in building nuclear facilities for Iraq's efforts to produce an atom bomb. "I felt that as an Arab, it was right that an Arab country have the bomb," he said. "Israel has one. So should we." He felt this way even though he said two of his cousins were executed by Hussein's security forces during the early 1980s for anti-government activities.

For all his pride, Ahmed was not fully trusted by his Iraqi overseers. No one was, he said. At first, he was told that his work was leading the way for nuclear-generated electric power. But eventually his bosses revealed the real goal.

In any case, after the Persian Gulf War in 1991, the nuclear arms project stopped, he said. U.N. inspectors came. Hiding the large infrastructure necessary to produce weapons-grade material was impossible.

Nonetheless, his supervisors warned Ahmed and his colleagues to "say little and answer only as narrowly as possible -- the specifics of our particular job, not what we knew about the whole program," he said. "We also had to sign a paper swearing that we had no documents in our private possession. If someone found out otherwise, they said we would be killed."

Ahmed said that he and other nuclear workers were given other jobs throughout Iraq, and eventually he landed at the Military Industrial Commission, which is responsible for constructing weapons factories and military installations. In 1995, he said he was ordered to help construct laboratories and vats at a place called Al Hakam, southwest of Baghdad.

U.N. inspectors were still looking for weapons programs, and they interviewed Ahmed three times, he said. "Each worker simply gave a narrow account of his job. In my case, I was just building a vat," he said.

Colleagues at other places told him they were ordered to bury equipment or to move it around on large trucks, sometimes for days at a time. "It was a giant chess game in which sometimes the pieces went underground," he said.

At Al Hakam, Ahmed said he asked his supervisor what the vats would be used for. Fermentation, he was told. When he asked what ingredient would be converted into what product, he was met with "aggressive silence." The Al Hakam facility was discovered only because of information provided by Hussein Kamel Hassan Majeed, a son-in-law of Hussein who defected to Jordan in the mid-1990s and conveyed information about the Iraqi biological weapons program.

Ahmed never knew whether the plant had produced a germ. All the time he was at Al Hakam, production was delayed by problems in procuring proper pumps and other equipment, he said.

He left in 1997 and applied to emigrate. The government, fearful of defectors, forced him to stay in Iraq for two more years. In that time, officials surmised, he would lose contact with the programs he worked on and have nothing to offer investigators abroad. "I was very careful to cut off all ties with my former work," he said. "I wanted to leave. I stayed completely isolated. I didn't want to know anything."

Although he has been out of Iraq almost three years, he keeps a low profile. "Who knows? Saddam might think I know something I don't and try to eliminate me," he said. "I will never feel safe."