Obeidi, who headed Saddam's uranium enrichment program until it was shut down in 1991, is the only Iraqi weapons scientist that the CIA is known to have brought to the United States after the invasion last year. The CIA also flew eight family members here in August 2003 and secretly set them up in three adjoining apartments in a leafy Virginia suburb close to downtown Washington.

But it is far less clear what happened to most of the 500 other scientists U.S. officials considered to be at the core of Saddam's programs to build missiles and nuclear, biological and chemical weapons. And Obeidi is speaking out to warn how easy it would be for someone else to build a nuclear weapon.

U.S. officials intercepted offers from Iran in recent months to hire several former Iraqi nuclear and missile scientists. None are known to have gone to Tehran, which Washington believes is trying to build a nuclear weapon. But U.S. officials also are concerned about the danger that remains inside Iraq.

"The immediate fear is the proximity of these scientists to the insurgents and terrorists in Iraq," said a U.S. official who travels frequently to Baghdad and spoke on condition of anonymity. "That has become a compelling issue for us."

The search for Iraqi scientists, and evidence of the weapons of mass destruction they once produced, will take center stage Wednesday when Charles A. Duelfer, head of the CIA-run Iraq Survey Group, appears before the Senate intelligence and armed services committees to present his final 1,500 page report on Iraq's long-defunct efforts to produce chemical, biological and nuclear weapons.

Duelfer's report is likely to spark renewed debate in the presidential campaign as President Bush and Sen. John F. Kerry trade charges over whether America needed to go to war in Iraq.

Duelfer has found no evidence that Baghdad resumed its nuclear arms program or churned out any chemical or germ agents for military weapons after 1991, officials said. Nor did Duelfer find evidence of ongoing programs to develop or produce such weapons before the 2003 war.

But Duelfer also has told colleagues that Saddam intended to mobilize his scientists to resume production of illicit arms if Iraq ever were free of U.N. inspections, trade sanctions and other international oversight. He found evidence of small clandestine laboratories, procurement of banned materials overseas, and work on illegal missiles and drone aircraft.

As many as eight of the 500 weapons scientists remain in custody in Iraq, and about 70 others work in two programs in Baghdad that the State Department set up last February to hire out-of-work weapons experts. Others are teaching, working for Iraq industries or government ministries, or have moved to Egypt, the United Arab Emirates and other Arab nations.

Many others, including Obeidi's two former top deputies, have simply vanished.

Obeidi, who met Saddam three times, says there is "no question" that the former Iraqi president wanted to revive his illicit weapons programs but adds that it isn't clear if the dictator knew his regime had no active programs to build them. Obeidi says Saddam was a "lunatic" whose grip on reality was increasingly unstable in the years before the war.

"He lived in a world of hallucination," Obeidi, a dapper man of 60, said over lunch. "You could see he was deceiving himself. But he not only fooled himself. He fooled the world."

Obeidi also fooled the world. With a master's degree from the Colorado School of Mines in Golden, Colo., and a doctorate from the University College of Swansea in Wales, he is an urbane and educated man. During the 1980s, he traveled across the United States and Europe to buy nuclear components, computer programs and research by lying about his plans.

In July 1987, Saddam's top deputy ordered Obeidi to direct a covert crash program to enrich uranium. By the time the Persian Gulf War began in January 1991, he had built a prototype centrifuge system capable of turning Iraq's small stockpile of reactor-enriched uranium into weapons-grade fuel for a crude atomic bomb. Obeidi suspects Saddam would have used it against Israel.

"We were so close to getting a bomb," Obeidi says now. "We were so close to getting tens or hundreds of bombs. To us, the sky was the limit. ... Looking back, the world was lucky."

So was Obeidi. When Iraq shuttered its nuclear program after the ceasefire that ended the 1991 war, Obeidi buried his centrifuge designs and several key components in a 50-gallon barrel under a lotus tree in his back yard.

Over the next few years, under strict Iraqi orders to conceal himself and all signs of his mothballed program, he repeatedly lied to U.N. inspectors, disguised and destroyed evidence, and once ran out the back door and hid for hours behind a wall to avoid a surprise U.N. raid.

But the International Atomic Energy Agency, the U.N. nuclear watchdog agency, unraveled the centrifuge program by 1995. Inspectors removed the enriched uranium stockpiles and exposed Iraq's global network of suppliers. They dismantled or destroyed all Iraq's nuclear infrastructure and material by 1998, and Iraq officials never tried to revive it.

After the war began last year, U.S. officials published a list of 55 most-wanted Iraqi officials, starting with Saddam, and put their names and faces on playing cards. Obeidi was No. 66 on a broader, classified list of 200 most-wanted Iraqis.

Fearful of arrest amid the post-invasion chaos, he reached out by satellite phone to David Albright, a former IAEA inspector in Washington. With his help, Obeidi ultimately bartered his buried barrel of nuclear documents and components - the only known remnants of Iraq's nuclear program - for CIA-sponsored sanctuary.

Albright, who now heads the non-profit Institute for Science and International Security, says Obeidi was "everybody's nightmare" because he was able to avoid detection despite buying crucial nuclear parts and research directly from universities and companies around the world.

But Albright says Obeidi never realized he could have built a crude bomb with the 30 kilograms of enriched uranium that Iraq possessed in the 1980s.

Beyond that, Obeidi would have needed needed to build more than 1,000 centrifuges - not just the 50 he planned - to enrich Iraq's supply of low-grade uranium to bomb-grade quality.

"Iraq was still many years from operating a centrifuge program," Albright said. Still, he added, "if a terrorist had the capability that Iraq had in 1991, you'd be deeply, deeply scared."

Obeidi now has co-written a book, "The Bomb in My Garden," about his career. He hopes to find a job to resume research on nanotechnology, the science of materials and systems so small they are measured by molecules or atoms.

And he warns that it's probably easier to build a nuclear bomb today than when he tried -- and nearly succeeded.

"The danger is really imminent," he said. "Someone today could be more clever than I was. The black market is still open. The technology is more efficient and more accessible. My work can be repeated, or accelerated. That is the horror."

A CIA spokesman declined comment on the case. Obeidi, who signed an agreement with the CIA that prohibits him from discussing the agency's role, says he relies on funds he brought from Iraq while he looks for work.

Obeidi is a self-described "a high-rev engine," a scientist of incessant curiosity and intense energy. He is proud of his work, though his dark eyes and mouth twitch nervously as he speaks. He repeatedly boasts that he once was "Saddam's nuclear mastermind," and expresses no regrets or remorse.

"It never crossed my mind to take the other path and be a nobody," he said.