Minutes earlier he had approached a photographer friend of mine on the street, saying he wanted to reach out to Washington with some important information about Saddam's nuclear program. It was a desperate move. He had tried contacting U.S. troops, but they had rebuffed him and threatened him with arrest if he showed up again. Now he wanted to know if I could use my satellite phone to help him.

At first I didn't know whether to believe him. But that night, at his urging, I dialed the Washington number of David Albright, a former American member of the United Nations weapons inspections team in Iraq. When I explained who had given me his name, the line went silent for a moment.

"You are actually talking to Obeidi?" Albright finally asked. "Where is he? What did he say?"

Albright had met Obeidi in Iraq in the 1990s, when the U.N. inspectors were dismantling Saddam's WMD programs. Saddam had kept Obeidi's identity secret longer than that of any other scientist, Albright said. If anyone could say for sure what had happened to Iraq's nuclear program, it was him.

The next day we dialed Albright from Obeidi's walled garden, and the two former adversaries exchanged a long series of pleasantries, exclaiming about how many years had passed since they'd last spoken and asking after each other's health. Then Obeidi repeated to Albright what he had told me -- that the Iraqi nuclear program had been dead since the start of U.N. weapons inspections in 1991. He spoke slowly, choosing his words with caution.

"David, there are some things the inspectors never found," he said. "I am speaking of some important materials and documents. But I am afraid of saying more until I can be sure of my safety."

At the end of the conversation, Albright promised to bring the case to the attention of the U.S. government and intelligence community. He cautioned us to be patient -- the Bush administration, he noted, didn't seem to have much of a plan for dealing with Saddam's WMD scientists.

So we waited. A dapper 59-year-old, Obeidi arrived every day to greet me wearing an elegant abiyaa robe. When he felt especially nervous, we met in clandestine locations: by lamplight at my translator's home or in the courtyard of an Iraqi acquaintance. At other times, we sat on plastic lawn chairs in his garden, trying to figure out how he could avoid arrest by U.S. troops, as his wife and daughters served us cookies and tea. Every now and again, he would drop hints about the secrets he wanted to reveal.

Then one day, he gestured toward a spot in the garden. Buried under the lotus tree next to his rosebushes a few feet from where we sat, he said, was the core of Saddam's nuclear quest: blueprints and prototype pieces for building centrifuges to enrich uranium to bomb grade. Twelve years earlier, he had buried them on orders from Saddam's son Qusay -- presumably, he said, to use them to restart a bomb program someday.

Obeidi dug up the cache a few days later. When he showed me the four prototypes, his hands shook. The machine parts looked alien, like pieces of a futuristic motorcycle, most of them small enough to fit inside a briefcase. He explained that these components and the three-foot-high stack of diagrams were still immensely valuable -- and immensely dangerous. They represented the core knowledge it would take to jump-start a covert bomb program, anywhere in the world.

This was why Obeidi was so anxious. On any given day he might be arrested by U.S. forces who would consider him a "bad guy," or killed by Saddam loyalists who would see him as a collaborator, or kidnapped by some other country interested in what he knew. The decision to come forward had been a hard one.

The news from Albright over the satellite phone was discouraging. U.S. intelligence on the ground was hopelessly disorganized, and there was no guarantee that American troops wouldn't imprison Obeidi even if he offered to help them. As the days wore on he felt the clock ticking, and sometimes his fear and exasperation would show through. "Why aren't they more interested in finding out what I have to offer?" he once asked in the textbook English he had learned as a student at the Colorado School of Mines in the 1960s. "I can answer many of their questions. Surely for a great nation like the United States, it is no big deal to offer me security in exchange for everything I want to divulge. Why don't they want to help me?"

I didn't have an answer. Just weeks earlier, before the invasion, President Bush had railed against Saddam for intimidating his WMD scientists and hiding them from inspectors. Colin Powell had appeared before the United Nations Security Council and warned that Obeidi's centrifuge program posed a threat to the world. It was hard to explain why, having gone to war ostensibly to get control of Iraq's dangerous knowledge, the United States was now doing so little to follow through.

IT'S NOT AS IF the administration hasn't talked about the danger posed by Saddam's WMD scientists. Whether Iraq had actual weapons or just "capabilities" didn't matter, it has long argued: Even mere capabilities could leak out to terrorist groups or the states that support them. During the presidential campaign, John Kerry and President Bush reached a rare point of agreement when both named the spread of nuclear weapons as the No. 1 danger facing the United States.

As it happens, Saddam's nuclear centrifuge program during the late 1980s was one of the most efficient covert nuclear efforts the world has ever seen. The scientists who pulled it off are very gifted men and women, many of whom are now out of work. Their names are still being kept secret by the international agencies familiar with their work. But a source close to one of those agencies recently said that of the 200-some scientists at the top of its nuclear list, all but three remain unaccounted for. In a country with porous borders, where everyone -- but especially those associated with the former regime -- is in danger every day, many experts say at least some scientists are bound to be tempted to sell their knowledge to the highest bidder. And as the Pakistani network exposed last year shows, the nuclear black market is alive and well.

"Weapons don't make themselves," says Anne Harrington, director of the Committee on International Security and Arms Control at the National Academies. "Somebody has to interpret how to take military doctrine and intent and make it real. Materials, particularly nuclear materials, are not something you scoop out of the dirt. The human element is critical in all of this."

Nobody knows how many Iraqi scientists may have been lured over the borders into Iran, Syria, or beyond. Nobody knows because no one is keeping tabs. But several observers agree that so little attention is being paid to Iraq's scientists, the war may actually have increased the chances of nuclear capabilities proliferating beyond the country's borders. Between its unemployed scientists and the disappearance of large amounts of WMD-related materials from former weapons sites, Iraq now poses a nightmare scenario, according to Ray McGovern, who spent 27 years analyzing intelligence for the CIA and afterward cofounded Veteran Intelligence Professionals for Sanity. "The danger is much more acute, both from the proliferation side and the terrorism side," McGovern says. "Before we invaded, there was no evidence that Iraq had any plan or incentive to proliferate. They didn't even have a current plan to develop WMDs. They just hadn't been doing it. Now, my God, we have a magnet attracting all manner of foreign jihadists to a place where the WMD expertise is suddenly unprotected. It just boggles the mind."

IRAQI SCIENTISTS have good reason to fear what might happen if they offer to cooperate with the United States. Obeidi's former boss and Saddam's top science adviser, General Amer al-Saadi, turned himself in to U.S. authorities just before I met Obeidi. He was promptly jailed and kept in custody for at least two years; a military spokesman told the Associated Press last year that the U.S. was also detaining up to a dozen other scientists. The chemist Mohammed Munim al-Izmerly -- also said to have worked on Iraq's former WMD programs -- was taken into custody for questioning in April 2003. Ten months later his body was dropped off in a U.S. body bag at a Baghdad hospital. He had been killed by a blow to the head.

In the weeks after the invasion, I got to know Obeidi quite well. He was no Dr. Strangelove. He loved science and the pure logic of an engineering challenge, and his eyes would light up when we talked about early Mesopotamian art or American history. He said he detested Saddam, and lamented how the Baathists had turned the best minds of his generation toward destructive ends. What he cared about more than anything was the welfare of his wife and four grown children. But as the U.S. occupation wore on, that seemed an increasingly elusive goal.

More than a month after our first meeting, our satellite phone calls had failed to produce any kind of safe-haven offer from Washington. Operatives from the Defense Intelligence Agency as well as the CIA had tracked Obeidi down through third parties, summoned him to their respective headquarters, and demanded that he surrender all he knew. The DIA agents threatened to imprison him, he told me, and then asked that he not speak to anyone at the CIA; soon afterward, the CIA sent armed agents to his home and took away a sample of his documents, promising to safeguard his family.

Then, early on the morning of June 3, 2003, more than a dozen soldiers jumped over Obeidi's garden wall, kicked in his front door, and put him and his family facedown on their living room floor at gunpoint. Obeidi's wife and children watched as he was handcuffed and put in a Humvee. Evidently, the Army had finally caught wind of Obeidi's significance -- and, just as evidently, the troops knew nothing of their own intelligence agencies' contacts with him.

Obeidi escaped the fate of his former boss when the CIA intervened with the Army and got him released. Knowing that he was a marked man, he decided that his only hope was to go public. He consented to an interview with CNN, and soon afterward the CIA whisked him and his family off to Kuwait, where he underwent weeks of interrogations.

On June 26, the CIA posted a press release about Obeidi's cache -- the most valuable WMD evidence the U.S. has yet obtained in Iraq -- on its official website. It also put up digital photos of the components and even one of the key centrifuge diagrams. The pictures, which Albright says could be "incredibly useful" to any regime trying to start a covert nuclear program, were online for almost a week -- long enough to be downloaded and made freely available on the Internet -- before the agency took them down. Literally buried for 12 years, some of Saddam's hoard of nuclear knowledge got out because of the U.S. government, not in spite of it.

OBEIDI NOW LIVES with eight family members in a U.S. city that he asked me not to name. His son and three daughters are learning English and looking for jobs, and he occasionally gives talks to groups of government officials. He seems more relaxed than he did when I first met him, as though he is finally able to shed some of the fear and pressure of life in Baghdad. But the thought of his former colleagues still weighs heavily on his mind. One day as we were eating falafel from plastic plates in the food court near his new American home, sitting anonymously among the shoppers, he asked me why he was still the only Iraqi scientist whom the United States had seen fit to take out of harm's way.

"There are a number of people who could be brought here, at least temporarily, and make positive contributions to this society," he said. "These are very educated and skillful scientists. Surely this great nation could absorb a few more talented people."

During the 1990s, the International Atomic Energy Agency and other watchdog groups compiled lists of key participants in Saddam's WMD programs. The IAEA roll call alone included about 2,000 names. One of the few that has been made public is that of Dr. Faris Abdul Aziz, a mild-mannered engineer who oversaw a staff of more than 200 working on the nuclear centrifuge program. I met him in Obeidi's garden, and he told me that in the days after the invasion, he had gone to Saddam's former Republican Palace to offer cooperation to the U.S. military on behalf of himself and other top nuclear scientists. But U.S. officials only wanted to know if he knew where Saddam was hiding and where they might find WMD stockpiles. They never asked him back for another interview. Today, no one seems to know where he is. "We've been trying to get in touch with these guys for months," Albright says. "But by now they're probably so jaded and suspicious that they want nothing to do with the U.S."

An even greater concern is the flight risk posed by scientists one level down: the technicians who have precise, hands-on knowledge of how to manufacture WMD components. Their expertise is priceless, especially to a covert program looking for engineers who know how to put the pieces together. A source with close ties to intelligence on the issue recently told me of the case of a female scientist who worked in Saddam's centrifuge program, most likely Dr. Widad Hattam al-Jabbouri. In the 1980s, Jabbouri had mastered one of the most troublesome aspects of the uranium-enriching machine: the magnetic upper bearing that holds the centrifuge rotor as it spins at supersonic speeds. Her expertise on classified magnet technology was deep, and extremely valuable. "From what we have learned she has ended up at a university in Syria," the source said. "Apparently the Syrians basically set up a refuge for senior scientists, especially those with Baathist connections, who couldn't get any work in Iraq."

This does not necessarily mean that Jabbouri is working on a weapons program in Damascus. The Syrian government has stated that it has no nuclear program, despite the suspicions of many international experts. But her move to Syria underscores how loose a grasp the U.S. has on Iraq's WMD knowledge.

"The proliferation risk is higher than it was before, and a chaotic situation means this technology is going to spread," says Robert Baer, who spent 21 years as a case officer with the CIA in the Middle East. If the administration had been serious about neutralizing Saddam's weapons program, he says, "the troops would have been securing equipment at weapons sites as they invaded, and they would have been looking for scientists.... It tells you that this war had nothing to do with WMDs."

SHORTLY AFTER the invasion of Iraq, Anne Harrington, then the deputy director of the Proliferation Threat Reduction Office of the State Department's Non-Proliferation Bureau, began planning a trip to Iraq to meet former WMD scientists and help them get to work on rebuilding the country. Harrington had a legendary track record of working with scientists from the former Soviet Union. In 1997, she had cut through the red tape of diplomacy and sent an email directly to the head of the State Research Center for Virology and Biotechnology in Siberia. The contact led to increased U.S. government funds to help former Soviet bioweapons scientists apply for civilian projects at home rather than sell their expertise on the black market.

"Anne believed this was the most important thing to do," says Carl Phillips, a biological weapons expert from Texas Tech University who signed on to help Harrington in Iraq. "She believed in going over and putting our boots on the ground to find these people, and she was fearless."

Harrington and Phillips proposed a $20 million plan to reach out to scientists in Baghdad. Their plan didn't go over well with the Pentagon, which at that point controlled the interim government of Iraq; Phillips remembers being told that as a condition for going, they had to agree not to make a formal request for the $20 million.

Once they got to Baghdad, Harrington was aghast at the scale of the looting. Her $20 million would be a mere drop in the bucket. "You can't just put somebody in a lab," she notes. "Not when they don't have a microscope."

In the end, even Harrington's drop in the bucket evaporated -- never mind that the State Department had made an official announcement allocating the $20 million -- and Harrington and Phillips had to make do with $2 million scraped together from emergency funds. Albright says responsibility for the reversal lies with John Bolton, then the State Department's undersecretary for arms control and international security. "All of this was going to land on Bolton's desk," he notes. "And he was in the camp that thinks all these scientists are criminals." Other programs to help Iraqi scientists -- including a Department of Energy program coordinated through Sandia National Laboratories in New Mexico -- have also come up short. "There are tens of thousands of scientists and engineers in need of a job," says Dr. Arian Pregenzer, a senior scientist at Sandia's Cooperative Monitoring Center. "We estimated it would be a $50-million-a-year project. That money has not materialized from anyplace."

Phillips ended up working on his own in Iraq, traveling in a civilian car to make contact with any WMD scientists he could find; so far, he's been able to set up a small center that employs eight former weapons researchers. Harrington, for her part, resigned from the State Department this past spring, partly in frustration over the lack of funds. "When the most we could squeeze out of the system was two $2 million grants," she says, "it made us sit back and scratch our heads a little bit and say, 'Didn't we go to war because they had people who could produce weapons of mass destruction?' It's a little difficult to square that circle."