PARIS, June 21 — Even before the fires were extinguished at the World Trade Center and the Pentagon, conspiracy theories began flooding the Internet. A few quickly spilled out of Web sites and were widely circulated by e-mail before fading into oblivion. One, however, has taken on a life of its own in France. It was turned into a book that has become the publishing sensation of the spring.

In the book, "L'Effroyable Imposture," or "The Horrifying Fraud," Thierry Meyssan challenges the entire official version of the Sept. 11 attacks.

He claims the Pentagon was not hit by a plane, but by a guided missile fired on orders of far right-wingers inside the United States government. Further, he says, the planes that struck the World Trade Center were not flown by associates of Osama bin Laden, but were programmed by the same government people to fly into the twin towers.

What really interests him, though, is what he sees as the conspiracy behind these actions. He contends that it was organized by right-wing elements inside the government who were planning a coup unless President Bush agreed to increase military spending and go to war against Afghanistan and Iraq to promote the conspirators' oil interests.

To achieve their goals, the theory goes, they blamed Osama bin Laden for Sept. 11 and later broadened their targets to include the "axis of evil," centered on Iraq.

The 235-page book has been universally ridiculed by the French news media, while its arguments have been dismantled point by point in "L'Effroyable Mensonge," or "The Horrifying Lie," a new book by two French journalists.

A Pentagon spokesman said, "There was no official reaction because we figured it was so stupid."

Yet in the past three months, Mr. Meyssan's book has sold more than 200,000 copies in France, placing it at the top of best-seller lists for several weeks. Foreign rights have also been sold in 16 countries (a Spanish version is already on sale), and Mr. Meyssan traveled to Abu Dhabi in the United Arab Emirates in April to present his arguments at a local university.

The book's French publisher, Éditions Carnot, said it would release an English version in the United States in July.

Mr. Meyssan said in an interview that he was surprised his book had so far provoked no major debate, but he was convinced that his message was being heard.

"Two-thirds of the hits on our Web site come from the United States," he said. "I'm not saying all my readers agree with me, but they recognize that the official American version of the attacks is idiotic. If we can't believe the official version, where do we stand?"

It is nonetheless puzzling why so many of the French have been willing to pay the equivalent of $17 for "The Horrifying Fraud." Is it a symptom of latent anti-Americanism? Is it a reflection of the French public's famous distrust of its own government and mainstream newspapers? Or has the French love of logic been tickled by the apparent Cartesian neatness of a conspiracy theory?

Certainly, after Sept. 11, some leftist intellectuals suggested that the United States had invited the attacks through its support for Israel. Others recalled that Islamic militants had been financed and armed by the United States to fight the Soviet occupation of Afghanistan in the 1980's. Yet, in this case, Libération and Le Monde, left-of-center newspapers with no love for the Bush administration, have led the assault on Mr. Meyssan's book.

"The pseudotheories of `The Horrifying Fraud' feed off the paranoid anti-Americanism that is one of the permanent components of the French political caldron," Gérard Dupuy wrote in an editorial in Libération. Edwy Plenel, news editor at Le Monde, wrote: "It is very grave to encourage the idea that something which is real is in fact fictional. It is the beginning of totalitarianism."

Guillaume Dasquié and Jean Guisnel, the authors of "The Horrifying Lie," favor a different explanation for the book's success. They write of France's "profound social and political sickness," which leads people to embrace the idea "that they are victims of plots, that the truth is hidden from them, that they should not believe official versions, but rather that they should demystify all expressions of power, whatever they might be."

Still, even if some French are susceptible to conspiracy theories, few had heard of the book until March 16, when Mr. Meyssan appeared on a popular Saturday evening television program on France 2, a government-owned but independently run channel. In the program, Mr. Meyssan was allowed to expound his theory without being challenged by the host. In the two weeks that followed, his book sold 100,000 copies.

Mr. Meyssan himself seems an unlikely purveyor of tall stories. A 44-year-old former theology student, he dabbled in leftist politics before forming a political research company, Réseau Voltaire, or Voltaire Network, in 1994.

The company's Web site (www .reseauvoltaire.com) adopted specific causes, like fighting homophobia and opposing Jean-Marie Le Pen's far-right National Front. Its investigative methods seemed thorough and objective.

In person too, Mr. Meyssan, a slim, wiry man with short hair and penetrating eyes, comes over as both serious and rational.

French journalists who had given some credibility to his Web site were all the more surprised, then, to find him building a vast conspiracy theory around the fact that photographs of the Sept. 11 attack showed no airplane parts in or near the smoldering gap in the Pentagon. This became the departure point for his book.

The line of reasoning that follows is a case study in how a conspiracy theory can be built around contradictions in official statements, unnamed "experts" and "professional pilots," unverified published facts, references to past United States policy in Cuba and Afghanistan, use of technical information, "revelations" about secret oil-industry maneuvers and, above all, rhetorical questions intended to sow doubts. At the end of each chapter, Mr. Meyssan presents his speculation as fact.

To gather his evidence, he worked mainly from articles, statements and speculation found on the Internet. He did not travel to the United States to interview any witnesses. Indeed, he dismisses the accounts of witnesses to the crash of the American Airlines Boeing 757 into the Pentagon.

"Far from believing their depositions, the quality of these witnesses only underlines the importance of the means deployed by the United States Army to pervert the truth," he said.

His "truth" is that no Muslims took part in the attacks "because the Koran forbids suicide." To his original claim that the Pentagon was bombed from the inside, he has now added his conviction that the building was struck by an air-to-ground missile fired by the United States Air Force. "This type of missile, seen from the side, would easily remind one of a small civilian airplane," he said.

In response, Mr. Dasquié and Mr. Guisnel said they traveled to Washington and interviewed 18 witnesses to the Pentagon crash.

They also have named experts explaining how the Boeing 757 could disappear inside the crater caused by the impact. Further, they identify several people mentioned only by their initials in Mr. Meyssan's acknowledgments, including a French Army officer currently on trial for treason and a middle-ranking intelligence officer.

The book has proved to be a windfall for Mr. Meyssan's publisher. More accustomed to publishing marginal books on subjects like the "false" American moon landing in 1969 and the latest "truth" about U.F.O.'s, Éditions Carnot can now boast of its first best seller.

Further, confident that this conspiracy theory will endure, Mr. Meyssan and Carnot have just published a 192-page annex, with new documents, photographs and theories. They call it "Le Pentagate."