WASHINGTON — The Bush administration's prewar claims that Saddam Hussein had built a fleet of trucks and railroad cars to produce anthrax and other deadly germs were based chiefly on information from a now-discredited Iraqi defector code-named "Curveball," according to current and former intelligence officials.

U.S. officials never had direct access to the defector and didn't even know his real name until after the war. Instead, his story was provided by German agents, and his file was so thick with detail that American officials thought it confirmed long-standing suspicions that the Iraqis had developed mobile germ factories to evade weapons inspections.

Curveball's story has since crumbled under doubts raised by the Germans and the scrutiny of U.S. arms hunters, who have come to see his code name as particularly apt, given the problems that plagued much prewar intelligence collection and analysis.

U.N. weapons inspectors first hypothesized that such trucks might exist, officials said. Then they asked former exile leader Ahmed Chalabi, a bitter enemy of Saddam, to help search for intelligence supporting their theory.

Soon after, a young chemical engineer emerged in a German refugee camp and claimed he had been hired out of Baghdad University to design and build bio-warfare trucks for the Iraqi army.

Based largely on his account, President Bush and his aides repeatedly warned of the shadowy germ trucks, dubbed "Winnebagos of Death" or "Hell on Wheels" in lurid news accounts, and they became a crucial part of the White House case for war — including Secretary of State Colin L. Powell's dramatic presentation to the U.N. Security Council just weeks before the war began.

Only later, U.S. officials said, the CIA discovered that the defector is the brother of one of Chalabi's top aides and began suspecting that he might have been coached to provide false information. Partly as a result, some U.S. intelligence officials and congressional investigators fear that the CIA inadvertently conjured up and then chased a phantom weapons system.

David Kay, who resigned in January as head of the CIA-led group created to find illicit weapons in Iraq, said that of all the intelligence failures in Iraq, the case of Curveball is particularly troubling.

"This is the one that's damning," he said. "This is the one that has the potential for causing the largest havoc in the sense that it really looks like a lack of due diligence and care in going forward."

Kay said in an interview that the defector "was absolutely at the heart of a matter of intense interest to us." But Curveball turned out to be an "out-and-out fabricator," he added.

Last May, the CIA announced it had found two of the suspect trucks in northern Iraq, but the agency later backtracked. However, in the absence of any other evidence to support its prewar claims, the Bush administration has continued to cling to the possibility that bio-warfare trucks might still exist.

Vice President Dick Cheney as recently as January referred to the trucks as "conclusive" proof that Iraq was producing weapons of mass destruction. CIA Director George J. Tenet later told a Senate committee that he called Cheney to warn him that the evidence was increasingly suspect.

Tenet gave the first hint of the underlying problem in a speech at Georgetown University on Feb. 5.

"I must tell you we are finding discrepancies in some claims made by human sources" about mobile bio-weapons production, he said. "Because we lack direct access to the most important sources on this question, we have as yet been unable to resolve the differences."

U.S. and British intelligence officials have acknowledged since the war that lies or distortions by Iraqi opposition groups in exile contributed to numerous misjudgments about Iraq's suspected weapons programs. Chalabi's Iraqi National Congress is blamed most often, but the rival Iraqi National Accord and various Kurdish groups also funneled dubious defectors to Western intelligence, officials say.

Still, the Curveball case may be uniquely damaging because no other credible defector provided firsthand confirmation that Iraq modified vehicles to produce germ agents, and no separate proof was found before or after the war. Iraqi officials interrogated since the war have all denied the program's existence.

The story of Curveball is now under close review by an internal panel at the CIA, as well as House and Senate oversight committees. All are seeking to determine why so much of the prewar intelligence now appears seriously flawed.

Richard J. Kerr, a former deputy CIA director who is leading the internal CIA review, defended the CIA's handling of the case. He said there were strong reasons to believe the vehicles existed because the defector's information was consistent with years of intelligence on Iraq's covert efforts to obtain chemical, biological and nuclear weapons.

"It was detailed and specific and made a lot of sense," Kerr said. He said the CIA believed Iraq was building and hiding banned weapons programs in civilian chemical and pharmaceutical facilities. "You get reporting on mobile production facilities . . . and you say it makes some sense."

Nor did Kerr fault the agency for relying so heavily on an anonymous source whom it could not interview. In this case, Germany's Federal Intelligence Service, known as the BND, repeatedly rejected CIA requests to meet Curveball to protect their source. But U.S. and German officials said the BND furnished its file on the defector to U.S. authorities, and at times, had him answer specific questions from U.S. intelligence.

"Intelligence is often based on information where you can't go back and talk to the source or verify it," Kerr said. "So you turn to the basic questions. 'Does it make sense? Is it logical? Does it appear he could have been at the right place at the right time to know these things?' " The defector met those tests, he said.

One focus of the ongoing investigations is whether the CIA should have known Curveball was not credible. A former U.S. official who has reviewed the classified file said the BND warned the CIA last spring that they had "various problems with the source." Die Zeit, a German news weekly, first reported the warning last August.

The official said the BND sent the warning after Powell first described the bio-warfare trucks in detail to the U.N. Security Council on Feb. 5, 2003. It's unclear whether the German warning about Curveball arrived before the war began on March 19.

"You can imagine the consternation it kicked off," the official said. "It suggested that what they'd (the Germans) been passing to us was false. They were backing away."

Mark Mansfield, a CIA spokesman, declined to comment Friday on that charge or any other questions about the case. An official at BND headquarters in Berlin, who spoke on the condition of anonymity, also declined to answer questions. "We believed that Iraq had these mobile biological facilities," the official said.

Although CIA reports previously had referred to the bio-warfare trucks, Powell's dramatic presentation at the U.N. put them in the spotlight.

Citing "eyewitness accounts," he called them "one of the most worrisome things that emerges from the thick intelligence file we have on Iraq."

"We have firsthand descriptions of biological weapons factories on wheels and on rails," Powell said. He showed what he called "highly detailed and extremely accurate" diagrams of how the trucks were configured, and warned they could spew enough anthrax or botulinus toxin "in a single month to kill thousands upon thousands of people."

But Kay, who sought to confirm Curveball's claims in Iraq after the war, said Powell's account was "disingenuous."

"If Powell had said to the Security Council: 'It's one source, we never actually talked to him, and we don't know his name,' as he's describing this, I think people would have laughed us out of court," added Kay.

Powell assured U.N. diplomats that two other Iraqi sources, who he said were "in a position to know," had corroborated the "eyewitness account." The CIA later said those reports arrived in December 2000 and mid-2002.

Kay said the debriefing files on the pair showed they never had direct contact with the bio-production trucks. "None of them claimed to have seen them," he said. "They said they were aware of the mobile program. They had heard there was a mobile program."

CIA files showed that another Iraqi defector, an engineer who had worked alongside Curveball, had specifically denied they had worked on such facilities, Kay said. Powell did not cite that defector, however.

The CIA last month acknowledged that a fourth defector that Powell cited at the U.N., a former major in Iraq's Intelligence Service, had lied when he said that Iraq had built mobile research laboratories to test biological agents. The Pentagon's Defense Intelligence Agency twice debriefed that defector in early 2002 and reported his claims. But it then concluded that he did not have firsthand information and probably was coached by Chalabi's exile group.

In May 2002, the DIA posted a "fabrication notice" on a classified computer network to warn other U.S. intelligence agencies that the defector had lied. But CIA officials said the notice was overlooked, and his information was cited in Powell's speech and the CIA's October 2002 National Intelligence Estimate to Congress.

The Curveball case began in 1992, when weapons inspectors from the U.N. Special Commission in Iraq, frustrated at their failure to find Iraq's germ weapon factories, wrote an internal report that speculated that Iraq could have hidden small, mobile versions in modified vans or trucks.

Based on that hypothesis, the U.N. weapons hunters, and U.S. intelligence analysts studying U-2 spy plane and high-altitude satellite images of Iraq, were instructed to watch for a potential "signature" of a germ factory on wheels—pairs of 35-foot trucks, working in tandem, parked parallel, with communications gear, high security and a water source.

Eavesdropping on Iraqi military communications had already proved that they were moving sensitive documents to avoid detection. U.N. inspectors also knew that Iraq used tanker trucks to fill chemical warheads on the battlefield in the 1980s, raising suspicions that it might also have produced chemical or biological agents in trucks.

In 1994, Israel's military intelligence passed word that Iraq was hiding poison factories in commercial trucks—red-and-white "Tip Top Ice Cream" trucks and green moving vans from "Sajida Transport," named for the dictator's wife.

The U.N. inspectors concluded that neither company existed, and some inspectors were skeptical about the whole idea.

Raymond A. Zilinskas, who helped inspect 61 biological facilities in Iraq in 1994, says he argued that bio-production trucks were difficult to build, dangerous to operate and hard to hide. "They just didn't make sense from a technical or a security viewpoint," he said.

But the theory gained new credence when Gen. Amir Saadi, then a senior Iraqi weapons official, told U.N. inspectors in August 1995 that he had proposed building germ-producing trucks and other mobile facilities in 1988, chiefly to avoid air attack, but that regime officials rejected his concept as impractical.

Saadi, who became science adviser to Saddam and chief liaison to U.N. inspectors before the war, turned himself in to U.S. forces in Baghdad on April 12, 2003, after telling German TV that Iraq had no illicit weapons. He remains in U.S. custody.

Saadi's 1995 statement rang alarms at the CIA and elsewhere, however. Intelligence reports soon referred to a possible series of three trucks that would operate as a single bio-agent factory. One truck would carry fermenters, another mixing and preparation tanks and the third, equipment to process and store the product.

U.N. inspectors stepped up their search in response. So did Western spy services.

In 1996, Holland's National Intelligence and Security Agency, known as the BVD, sent word that an informant codenamed "Fulcrum," a former Iraqi intelligence officer, had supplied a list of government-issued, blue-and-white, sequentially numbered license plates that supposedly were used on the germ trucks. But the inspectors could never find licenses with those numbers.

Then, in March 1997, a U-2 spy plane that the U.S. government operated for the U.N. photographed three or four large box-type trucks parked outside a garage used by Iraq's Intelligence Service, the Mukhabarat. U.N. teams swooped in—and found the trucks were filled with construction material.

The U.N. team then asked headquarters in New York to let them run random roadblocks in Iraq. They also asked for "hot pursuit" authority, complete with fast cars and helicopters capable of spraying foam on the roads, in case they had to chase a fleeing germ-truck. Officials in New York quickly rejected both proposals.

"We were told that was insane," said Scott Ritter, a former chief U.N. inspector who headed a special investigations unit and who served as the U.N. team's liaison to U.S. intelligence. "And they were right."

But the U.N. operation in New York, then headed by Australian diplomat Richard Butler, did approve another plan.

The inspectors long had relied on intelligence from sympathetic governments and dissident groups. Chalabi, a prominent exile leader, had lobbied Washington for years to overthrow Saddam's regime and claimed he had spies inside the Baghdad regime.

In December 1997, Ritter said, he and his deputy, a former British army major attached to the U.N. team, flew to London to ask Chalabi for help.

"Chalabi outlined what he could do for us," Ritter recalled. "His intelligence guy outlined their sources and said he had people inside the government. They told us they had the run of Iraq. Just tell them what we needed. So we outlined the gaps in our understanding of the Iraqi program, including the mobile bio-weapons labs. Basically, we gave them a shopping list."

"They began feeding us information," Ritter said. "We got hand-drawn maps, handwritten statements and other stuff flowing in. At first blush, it looked good. But nothing panned out. Most of it just regurgitated what we'd given them. And the data that was new never checked out."

Haider Moosawi, an INC media liaison in Baghdad, said in a telephone interview Saturday that he could not confirm the meetings had occurred. Asked about INC ties to Curveball, he replied, "I really can't think of such a defector."

U.S. officials say Curveball apparently showed up in Germany in 1998, but it is unclear how he got there. The Los Angeles Times was unable to confirm Curveball's real name or his current location.

What is clear is that by 2000, Curveball had provided a vast array of convincing detail about the illicit program he claimed to manage.

He outlined how each office was set up and the names on each door. He described how walls were moved to help hide trucks. He identified several dozen fellow team members—even a lowly aide who rented their cars. He diagramed how stainless steel tanks, pumps, compressors and other parts were configured on nickel-plate flooring in each truck.

U.N. weapons hunters who returned to Iraq in November 2002 considered the trucks a "high priority," said a former inspector who helped supervise more than 70 raids for evidence of germ weapons in the four months before the war.

They checked every site Curveball had identified, as well as others picked by U.S. intelligence. They tested waste lines in food-testing vans, took samples from refrigerator trucks, and searched for truck parts, blueprints, purchase orders or other evidence in factories, laboratories and elsewhere.

"We didn't find anything," the former inspector said.

After Powell's dramatic U.N. speech, inspectors demanded Baghdad identify every mobile facility it owned.

In letters delivered on March 3 and March 15, just days before the war started, Iraqi officials handed over detailed descriptions, backed by 39 photographs and four videotapes, of mobile disease analysis labs, mobile military morgues, X-ray trucks, military bakery vans, mobile ice factories, refrigerated drug and food transport trucks and other special vehicles. Some had stainless-steel equipment that appeared similar to the diagrams Powell had shown the United Nations.

After major combat ended, the U.S. forces recovered two suspect trailer-trucks in northern Iraq. A CIA report last May 28 concluded that two trucks "probably" were designed to produce lethal toxins in liquid slurry, and President Bush said U.S. forces thus had "found" Iraq's weapons of mass destruction.

But Pentagon analysts warned that the trucks probably produced hydrogen for artillery weather balloons, and the CIA backtracked. It now says there is "no consensus" on the trucks' use.

Kay's investigators visited Curveball's parents and brother in Baghdad, as well as his former work sites. They determined that he was last in his class at the University of Baghdad, not first as he had claimed. They learned he had been fired from his job and jailed for embezzlement before he fled Iraq.

"He was wrong about so much," Kay recalled. "Physical descriptions he gave for buildings and sites simply didn't match reality. Things started to fall apart."

Chalabi, now a member of the Iraqi Governing Council, retains strong support in the White House. He was a guest of first lady Laura Bush at the president's State of the Union Speech last January, and his organization still receives several hundred thousand dollars a month from the Pentagon to help collect intelligence in Iraq.

Chalabi says he has been unfairly blamed for the failure to find germ trucks or any other unconventional weapons in Iraq since the war. He blames the CIA instead.

"Intelligence people are supposed to do a better job for their country, and their government did not do such a good job," he told CBS' "60 Minutes" in a recent interview. "This is a ridiculous situation," he added.

INC defectors were always accused of having an ax to grind, he said. "So why did the CIA believe them so much?"

Contributing: Jeffrey Fleishman.