The FBI has maintained that coincidence is the best way to explain John Doe No. 2, whose character sketch was drawn mainly from the account of an eyewitness at the Junction City shop where the Ryder truck was rented. That witness, the FBI says, mixed up his recollections and mistakenly identified a man who came in the next day to rent a truck -- a 23-year-old soldier named Todd Bunting -- as an accomplice of McVeigh's. Bunting, who was cleared of any connection to the crime, vaguely resembled the composite drawing and wore clothes similar to those in the drawing, including a Carolina Panthers ball cap.

There is a kind of logic to the FBI's conclusion. The Oklahoma City case was anything but straightforward, and the agency was hit with a near-apocalyptic flood of tips about the possible perpetrators of the bombing. The vast majority of them turned into time-wasting dead ends and wild goose chases, and the investigators were forced to turn to Occam's Razor -- the maxim that the simplest explanation for a mystery is most often the correct one -- to shave down the possibilities.

McVeigh, a dead ringer for the John Doe No. 1 sketch, had been captured, and Terry Nichols (who looked nothing like John Doe No. 2) had turned himself in to authorities. The Fortiers were quickly tracked down and confessed to their relatively minor roles in the bombing as sympathizers who gave McVeigh a temporary base of operations and listened avidly as he planned the attack. And though there was no shortage of theories about the identity of Doe No. 2, no one who resembled him emerged as a possible co-conspirator.

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Ultimately, investigators were forced to conclude that John Doe No. 2 was a phantom who never really existed. And that was the case they chose to take to the courts in their prosecutions of McVeigh and Nichols.

"There's nothing there," says FBI spokesman Steven Berry. "It's a case where every avenue we went down, there's nothing there. And we're certainly not going to get behind it and say there's something there or put it out that there is something when there's nothing there. It's chasing ghosts."

Indeed, McVeigh himself steadfastly denies there was any John Doe No. 2. He told the authors of "American Terrorist" that he and Nichols alone had built and detonated the bomb and vehemently denied that anyone else had been involved. He also denied the existence of Doe No. 2 in a May 2 letter to the Houston Chronicle.

But even McVeigh's own trial attorney, Stephen Jones, never believed him on this count. Jones believes McVeigh had substantial motive to lie about the involvement of others: For one, it covers the tracks of his cohorts, and it heightens his own role in the drama. Certainly "American Terrorist" captures McVeigh's desire for martyrdom -- he manipulated his appeals to expedite his execution -- and admitting anyone else into the scenario would certainly diminish his starring role.

Jones also told reporters that McVeigh failed a lie-detector test when asked about John Doe No. 2. And McVeigh, he says, frequently covered up any traces of potential co-conspirators. Once he insisted he had not accompanied Nichols to a farm co-op to buy ammonium nitrate, but after learning that a clerk at the store identified Nichols and said there was a second man with him, McVeigh flip-flopped, telling Jones he had been the man there after all. The clerk, on the other hand, insisted that it hadn't been McVeigh.

But when Jones' defense team attempted to track down Doe No. 2, it ran into the same dead ends as the FBI. Nonetheless, Jones himself came to believe McVeigh was associated with a gang of white supremacists operating out of an enclave in rural Missouri called Elohim City.

That theory is also a favorite of conspiracists who see the Oklahoma City investigation as a massive coverup. Many of them go well beyond Jones' relatively modest conjectures about the nature of the bombing to argue that the government itself was somehow involved in the bombing, as part of its plan to discredit the militia movement. The theory that McVeigh was set up looms large in the voluminous conspiracy theories that are the metier of the far-right Patriot movement. The Militia of Montana, for instance, continues to claim that there was a second blast -- a charge set by federal agents, they say -- recorded within seconds of the truck bomb (there was not; the seismic reports that form the basis of this claim actually recorded the impact of the mass of debris from the Murrah Building hitting the ground).

Others argue that a bomb made of ammonium nitrate and fuel oil could not have delivered enough force to cause the extraordinary damage of the Oklahoma City blast, and cite a study at a federal laboratory as proof. They are right. But then, the explosion set off by McVeigh actually was a high-octane mix of jet fuel and fertilizer, and the Murrah damage was entirely consistent with the force of that kind of bomb.

The theories that have gained the most currency among the conspiracy set are traceable to an Oklahoma journalist named J.D. Cash, who has built a minor career out of linking McVeigh's activities back to Elohim City and other violent supremacist factions. The core of Cash's theories revolve around McVeigh's connections to a handful of people at Elohim City who shared anti-government (and deeply racist) views, suggesting that McVeigh and his co-conspirators were actually dupes of a federal informant acting as an agent provocateur.

However, Cash's theories crumble in the face of a careful examination of the facts of the case. Cash makes much of the shadowy presence of a German neo-Nazi named Andreas Strassmeier and McVeigh's attempts to contact him at Elohim City in the days before the bombing. But Strassmeier had little contact with McVeigh and was nowhere near any of the activities that produced the bomb, and he steadfastly denies any connection. Cash's chief witness, an ex-debutante turned white-power pinup girl named Carol Howe who eventually worked as a Bureau of Alcohol, Tobacco and Firearms informant, has constantly changed her story in a way seeming to indicate that she was tailoring it to suit the needs of the conspiracists who promoted her tale.

These theories reached a kind of apex in the work of a British journalist named Ambrose Evans-Pritchard, whose 1997 book, "The Secret Life of Bill Clinton," postulated that the former president covered up the government's complicity in the bombing as part of a larger career of perfidy that included drug-running and murder. Though Evans-Pritchard's work gained some favor among mainstream conservatives -- Robert Novak, for instance, wrote a column extolling his theories -- nearly every aspect of "Secret Life" has been roundly debunked.

Cash's work surfaced again recently as a source for a report by the British newspaper The Guardian that linked McVeigh's activities to those of the Aryan Republican Army, a gang of Midwestern bank robbers whose whereabouts eerily paralleled those of McVeigh at key moments in the run-up to the bombing. However, like nearly everything proceeding from Cash, the piece was built on a fabric of coincidence and speculation.

Indeed, there has been no shortage of candidates for the identity of John Doe No. 2, but nearly all of them lead to the same kind of factual dead ends. And it is precisely those failures that tend to bolster the government's contention that the man in the sketch never existed as an actual conspirator in the bombing.

But the FBI's explanation of the John Doe No. 2 theories is nearly as full of holes as the conspiracists' scenarios -- or at least, it leaves dangling a long list of unanswered questions. When it is examined, a troubling portrait emerges of an agency eager to tailor its investigation for the purposes of prosecuting a criminal case, rather than doggedly seeking out the truth.

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