If the CIA had a smoking gun, evidence of Saddam Hussein's mobile weapons labs was it. The agency's first tip-off came in 2000. "The source was an eyewitness, an Iraqi chemical engineer who had supervised one of these facilities," Secretary of State Colin Powell told the world in his address to the U.N. Security Council on Feb. 5, 2003. Elisa Harris, who was in charge of the WMD brief at the National Security Council in 2000, doesn't remember the report as being that detailed. But the tip was "worrisome," she says. There seemed to be evidence to suggest Iraq was reconstituting a program to both make and spread so-called BW--biological weapons.

In 2001 a second informant emerged: an Iraqi engineer, as Powell later described him. The CIA hardened its view, but still hedged in its white paper in the second half of 2001. "Baghdad continues to a pursue a BW program," the report said, expressing concern about "the likely availability of mobile covert facilities." In 2002 two more informants on mobile labs turned up, one of them a "major" who defected. Now the CIA's 2002 National Intelligence Estimate took on a tone of certainty: Baghdad "has established a large-scale... BW production capability, which includes mobile facilities." Citing the four informants, Powell told the U.N.: "The description our sources gave us of the technical features required by such facilities is highly detailed and extremely accurate." When three suspicious tractor-trailers were found in Iraq after the war, the CIA crowed that its intelligence had been solid.

Ten months later, all that was once solid seems to be melting away. Like so much else about Saddam's elusive WMD, the mystery of the mobile labs has only deepened. "There is no consensus within our community over whether the trailers were for [weapons] use or if they were used for the production of hydrogen," CIA Director George Tenet admitted in a speech last week. David Kay, until recently head of the agency's Iraq Survey Group, says he believes the trailers weren't used for weapons at all.

So how did the trucks become one of the most compelling briefs in America's case for war? The transformation of the mobile-lab intel from speculation to fact is a case study in the enduring fallibility of "humint," human intel-gathering--and how U.S. agencies fail to communicate. NEWSWEEK has learned that as early as May 2002, analysts at the Pentagon's Defense Intelligence Agency issued a warning about the credibility of one of the mobile-lab informants, believed to be the Iraqi major. According to an official who has read the still-secret warning, known as a "fabricator notice," the document reported that he had been "coached by Iraqi National Congress," an exile group eagerly pushing to Pentagon hawks the urgent need to depose Saddam.

Intelligence officials say the warning notice was widely circulated within intelligence agencies, including the DIA and CIA, before the defector's story was included in Powell's speech. But apparently the warning was not properly cross-referenced in intelligence computers. As a result, officials explained, the analysts drawing up administration papers and speeches on Saddam's WMD continued to use the original reports. A Powell aide said the secretary learned of the glitch only in the past few weeks. One official said that at least one other of the four Iraqi informants cited by Powell is also now considered to be less than reliable.

The bottom line is that the CIA was relying on informants--and little else. The genesis of the tale of the mobile labs is now being examined by Senate investigators. The informants' stories are also likely to provide fodder for a new bipartisan commission, announced by President George W. Bush last week, to examine the flawed intel on Iraq, and the larger issue of why America's intelligence community seems to get so much so wrong.

The CIA insists that its main source--the chemical engineer--is still credible. Officials say he has nothing to do with the INC and they insist the trucks found in Kurdistan could still be mobile germ-warfare factories (though intelligence officials acknowledge they are less certain now). Tenet insisted last week that the CIA still gets a lot right. But that's hardly a reassuring case for the next pre-emptive war.