Last September 24th, as Congress prepared to vote on the resolution authorizing President George W. Bush to wage war in Iraq, a group of senior intelligence officials, including George Tenet, the Director of Central Intelligence, briefed the Senate Foreign Relations Committee on Iraq’s weapons capability. It was an important presentation for the Bush Administration. Some Democrats were publicly questioning the President’s claim that Iraq still possessed weapons of mass destruction which posed an immediate threat to the United States. Just the day before, former Vice-President Al Gore had sharply criticized the Administration’s advocacy of preëmptive war, calling it a doctrine that would replace “a world in which states consider themselves subject to law” with “the notion that there is no law but the discretion of the President of the United States.” A few Democrats were also considering putting an alternative resolution before Congress.

According to two of those present at the briefing, which was highly classified and took place in the committee’s secure hearing room, Tenet declared, as he had done before, that a shipment of high-strength aluminum tubes that was intercepted on its way to Iraq had been meant for the construction of centrifuges that could be used to produce enriched uranium. The suitability of the tubes for that purpose had been disputed, but this time the argument that Iraq had a nuclear program under way was buttressed by a new and striking fact: the C.I.A. had recently received intelligence showing that, between 1999 and 2001, Iraq had attempted to buy five hundred tons of uranium oxide from Niger, one of the world’s largest producers. The uranium, known as “yellow cake,” can be used to make fuel for nuclear reactors; if processed differently, it can also be enriched to make weapons. Five tons can produce enough weapon-grade uranium for a bomb. (When the C.I.A. spokesman William Harlow was asked for comment, he denied that Tenet had briefed the senators on Niger.)

On the same day, in London, Tony Blair’s government made public a dossier containing much of the information that the Senate committee was being given in secret—that Iraq had sought to buy “significant quantities of uranium” from an unnamed African country, “despite having no active civil nuclear power programme that could require it.” The allegation attracted immediate attention; a headline in the London Guardian declared, “african gangs offer route to uranium.”

Two days later, Secretary of State Colin Powell, appearing before a closed hearing of the Senate Foreign Relations Committee, also cited Iraq’s attempt to obtain uranium from Niger as evidence of its persistent nuclear ambitions. The testimony from Tenet and Powell helped to mollify the Democrats, and two weeks later the resolution passed overwhelmingly, giving the President a congressional mandate for a military assault on Iraq.

On December 19th, Washington, for the first time, publicly identified Niger as the alleged seller of the nuclear materials, in a State Department position paper that rhetorically asked, “Why is the Iraqi regime hiding their uranium procurement?” (The charge was denied by both Iraq and Niger.) A former high-level intelligence official told me that the information on Niger was judged serious enough to include in the President’s Daily Brief, known as the P.D.B., one of the most sensitive intelligence documents in the American system. Its information is supposed to be carefully analyzed, or “scrubbed.” Distribution of the two- or three-page early-morning report, which is prepared by the C.I.A., is limited to the President and a few other senior officials. The P.D.B. is not made available, for example, to any members of the Senate or House Intelligence Committees. “I don’t think anybody here sees that thing,” a State Department analyst told me. “You only know what’s in the P.D.B. because it echoes—people talk about it.”

President Bush cited the uranium deal, along with the aluminum tubes, in his State of the Union Message, on January 28th, while crediting Britain as the source of the information: “The British government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa.” He commented, “Saddam Hussein has not credibly explained these activities. He clearly has much to hide.”

Then the story fell apart. On March 7th, Mohamed ElBaradei, the director-general of the International Atomic Energy Agency, in Vienna, told the U.N. Security Council that the documents involving the Niger-Iraq uranium sale were fakes. “The I.A.E.A. has concluded, with the concurrence of outside experts, that these documents . . . are in fact not authentic,” ElBaradei said.

One senior I.A.E.A. official went further. He told me, “These documents are so bad that I cannot imagine that they came from a serious intelligence agency. It depresses me, given the low quality of the documents, that it was not stopped. At the level it reached, I would have expected more checking.”

The I.A.E.A. had first sought the documents last fall, shortly after the British government released its dossier. After months of pleading by the I.A.E.A., the United States turned them over to Jacques Baute, who is the director of the agency’s Iraq Nuclear Verification Office.

It took Baute’s team only a few hours to determine that the documents were fake. The agency had been given about a half-dozen letters and other communications between officials in Niger and Iraq, many of them written on letterheads of the Niger government. The problems were glaring. One letter, dated October 10, 2000, was signed with the name of Allele Habibou, a Niger Minister of Foreign Affairs and Coöperation, who had been out of office since 1989. Another letter, allegedly from Tandja Mamadou, the President of Niger, had a signature that had obviously been faked and a text with inaccuracies so egregious, the senior I.A.E.A. official said, that “they could be spotted by someone using Google on the Internet.”

The large quantity of uranium involved should have been another warning sign. Niger’s “yellow cake” comes from two uranium mines controlled by a French company, with its entire output presold to nuclear power companies in France, Japan, and Spain. “Five hundred tons can’t be siphoned off without anyone noticing,” another I.A.E.A. official told me.

This official told me that the I.A.E.A. has not been able to determine who actually prepared the documents. “It could be someone who intercepted faxes in Israel, or someone at the headquarters of the Niger Foreign Ministry, in Niamey. We just don’t know,” the official said. “Somebody got old letterheads and signatures, and cut and pasted.” Some I.A.E.A. investigators suspected that the inspiration for the documents was a trip that the Iraqi Ambassador to Italy took to several African countries, including Niger, in February, 1999. They also speculated that MI6—the branch of British intelligence responsible for foreign operations—had become involved, perhaps through contacts in Italy, after the Ambassador’s return to Rome.

Baute, according to the I.A.E.A. official, “confronted the United States with the forgery: ‘What do you have to say?’ They had nothing to say.”

ElBaradei’s disclosure has not been disputed by any government or intelligence official in Washington or London. Colin Powell, asked about the forgery during a television interview two days after ElBaradei’s report, dismissed the subject by saying, “If that issue is resolved, that issue is resolved.” A few days later, at a House hearing, he denied that anyone in the United States government had anything to do with the forgery. “It came from other sources,” Powell testified. “It was provided in good faith to the inspectors.”