By WILLIAM J. BROAD

Correction Appended

Last March, the federal government set up a Web site to make public a vast archive of Iraqi documents captured during the war. The Bush administration did so under pressure from Congressional Republicans who had said they hoped to “leverage the Internet” to find new evidence of the prewar dangers posed by Saddam Hussein.

But in recent weeks, the site has posted some documents that weapons experts say are a danger themselves: detailed accounts of Iraq’s secret nuclear research before the 1991 Persian Gulf war. The documents, the experts say, constitute a basic guide to building an atom bomb.

Last night, the government shut down the Web site after The New York Times asked about complaints from weapons experts and arms-control officials. A spokesman for the director of national intelligence said access to the site had been suspended “pending a review to ensure its content is appropriate for public viewing.”

Officials of the International Atomic Energy Agency, fearing that the information could help states like Iran develop nuclear arms, had privately protested last week to the American ambassador to the agency, according to European diplomats who spoke on condition of anonymity because of the issue’s sensitivity. One diplomat said the agency’s technical experts “were shocked” at the public disclosures.

Early this morning, a spokesman for Gregory L. Schulte, the American ambassador, denied that anyone from the agency had approached Mr. Schulte about the Web site.

The documents, roughly a dozen in number, contain charts, diagrams, equations and lengthy narratives about bomb building that nuclear experts who have viewed them say go beyond what is available elsewhere on the Internet and in other public forums. For instance, the papers give detailed information on how to build nuclear firing circuits and triggering explosives, as well as the radioactive cores of atom bombs.

“For the U.S. to toss a match into this flammable area is very irresponsible,” said A. Bryan Siebert, a former director of classification at the federal Department of Energy, which runs the nation’s nuclear arms program. “There’s a lot of things about nuclear weapons that are secret and should remain so.”

The government had received earlier warnings about the contents of the Web site. Last spring, after the site began posting old Iraqi documents about chemical weapons, United Nations arms-control officials in New York won the withdrawal of a report that gave information on how to make tabun and sarin, nerve agents that kill by causing respiratory failure.

The campaign for the online archive was mounted by conservative publications and politicians, who said that the nation’s spy agencies had failed adequately to analyze the 48,000 boxes of documents seized since the March 2003 invasion. With the public increasingly skeptical about the rationale and conduct of the war, the chairmen of the House and Senate intelligence committees argued that wide analysis and translation of the documents — most of them in Arabic — would reinvigorate the search for clues that Mr. Hussein had resumed his unconventional arms programs in the years before the invasion. American search teams never found such evidence.

The director of national intelligence, John D. Negroponte, had resisted setting up the Web site, which some intelligence officials felt implicitly raised questions about the competence and judgment of government analysts. But President Bush approved the site’s creation after Congressional Republicans proposed legislation to force the documents’ release.

In his statement last night, Mr. Negroponte’s spokesman, Chad Kolton, said, “While strict criteria had already been established to govern posted documents, the material currently on the Web site, as well as the procedures used to post new documents, will be carefully reviewed before the site becomes available again.”

A spokesman for the National Security Council, Gordon D. Johndroe, said, “We’re confident the D.N.I. is taking the appropriate steps to maintain the balance between public information and national security.”

The Web site, “Operation Iraqi Freedom Document Portal,” was a constantly expanding portrait of prewar Iraq. Its many thousands of documents included everything from a collection of religious and nationalistic poetry to instructions for the repair of parachutes to handwritten notes from Mr. Hussein’s intelligence service. It became a popular quarry for a legion of bloggers, translators and amateur historians.

Among the dozens of documents in English were Iraqi reports written in the 1990s and in 2002 for United Nations inspectors in charge of making sure Iraq had abandoned its unconventional arms programs after the Persian Gulf war. Experts say that at the time, Mr. Hussein’s scientists were on the verge of building an atom bomb, as little as a year away.

European diplomats said this week that some of those nuclear documents on the Web site were identical to the ones presented to the United Nations Security Council in late 2002, as America got ready to invade Iraq. But unlike those on the Web site, the papers given to the Security Council had been extensively edited, to remove sensitive information on unconventional arms.

The deletions, the diplomats said, had been done in consultation with the United States and other nuclear-weapons nations. Mohamed ElBaradei, the director of the International Atomic Energy Agency, which ran the nuclear part of the inspections, told the Security Council in late 2002 that the deletions were “consistent with the principle that proliferation-sensitive information should not be released.”

In Europe, a senior diplomat said atomic experts there had studied the nuclear documents on the Web site and judged their public release as potentially dangerous. “It’s a cookbook,” said the diplomat, who spoke on condition of anonymity because of his agency’s rules. “If you had this, it would short-circuit a lot of things.”

The New York Times had examined dozens of the documents and asked a half dozen nuclear experts to evaluate some of them.

Peter D. Zimmerman, a physicist and former United States government arms scientist now at the war studies department of King’s College, London, called the posted material “very sensitive, much of it undoubtedly secret restricted data.”

Ray E. Kidder, a senior nuclear physicist at the Lawrence Livermore National Laboratory in California, an arms design center, said “some things in these documents would be helpful” to nations aspiring to develop nuclear weapons and should have remained secret.

A senior American intelligence official who deals routinely with atomic issues said the documents showed “where the Iraqis failed and how to get around the failures.” The documents, he added, could perhaps help Iran or other nations making a serious effort to develop nuclear arms, but probably not terrorists or poorly equipped states. The official, who requested anonymity because of his agency’s rules against public comment, called the papers “a road map that helps you get from point A to point B, but only if you already have a car.”

Thomas S. Blanton, director of the National Security Archive, a private group at George Washington University that tracks federal secrecy decisions, said the impetus for the Web site’s creation came from an array of sources — private conservative groups, Congressional Republicans and some figures in the Bush administration — who clung to the belief that close examination of the captured documents would show that Mr. Hussein’s government had clandestinely reconstituted an unconventional arms programs.

“There were hundreds of people who said, ‘There’s got to be gold in them thar hills,’ ” Mr. Blanton said.

The campaign for the Web site was led by the chairman of the House Intelligence Committee, Representative Peter Hoekstra of Michigan. Last November, he and his Senate counterpart, Pat Roberts of Kansas, wrote to Mr. Negroponte, asking him to post the Iraqi material. The sheer volume of the documents, they argued, had overwhelmed the intelligence community.

Some intelligence officials feared that individual documents, translated and interpreted by amateurs, would be used out of context to second-guess the intelligence agencies’ view that Mr. Hussein did not have unconventional weapons or substantive ties to Al Qaeda. Reviewing the documents for release would add an unnecessary burden on busy intelligence analysts, they argued.

On March 16, after the documents’ release was approved, Mr. Negroponte’s office issued a terse public announcement including a disclaimer that remained on the Web site: “The U.S. government has made no determination regarding the authenticity of the documents, validity or factual accuracy of the information contained therein, or the quality of any translations, when available.”

On April 18, about a month after the first documents were made public, Mr. Hoekstra issued a news release acknowledging “minimal risks,” but saying the site “will enable us to better understand information such as Saddam’s links to terrorism, weapons of mass destruction and violence against the Iraqi people.” He added: “It will allow us to leverage the Internet to enable a mass examination as opposed to limiting it to a few exclusive elites.”

Yesterday, before the site was shut down, Jamal Ware, a spokesman for Mr. Hoekstra, said the government had “developed a sound process to review the documents to ensure sensitive or dangerous information is not posted.” Later, he said the complaints about the site “didn’t sound like a big deal,” adding, “We were a little surprised when they pulled the plug.”

The precise review process that led to the posting of the nuclear and chemical-weapons documents is unclear. But in testimony before Congress last spring, a senior official from Mr. Negroponte’s office, Daniel Butler, described a “triage” system used to sort out material that should remain classified. Even so, he said, the policy was to “be biased towards release if at all possible.” Government officials say all the documents in Arabic have received at least a quick review by Arabic linguists.

Some of the first posted documents dealt with Iraq’s program to make germ weapons, followed by a wave of papers on chemical arms.

At the United Nations in New York, the chemical papers raised alarms at the Monitoring, Verification and Inspection Commission, which had been in charge of searching Iraq for all unconventional arms, save the nuclear ones.

In April, diplomats said, the commission’s acting chief weapons inspector, Demetrius Perricos, lodged an objection with the United States mission to the United Nations over the document that dealt with the nerve agents tabun and sarin.

Soon, the document vanished from the Web site. On June 8, diplomats said, Mr. Perricos told the Security Council of how risky arms information had shown up on a public Web site and how his agency appreciated the American cooperation in resolving the matter.

In September, the Web site began posting the nuclear documents, and some soon raised concerns. On Sept. 12, it posted a document it called “Progress of Iraqi nuclear program circa 1995.” That description is potentially misleading since the research occurred years earlier.

The Iraqi document is marked “Draft FFCD Version 3 (20.12.95),” meaning it was preparatory for the “Full, Final, Complete Disclosure” that Iraq made to United Nations inspectors in March 1996. The document carries three diagrams showing cross sections of bomb cores, and their diameters.

On Sept. 20, the site posted a much larger document, “Summary of technical achievements of Iraq’s former nuclear program.” It runs to 51 pages, 18 focusing on the development of Iraq’s bomb design. Topics included physical theory, the atomic core and high-explosive experiments. By early October, diplomats and officials said, United Nations arms inspectors in New York and their counterparts in Vienna were alarmed and discussing what to do.

Last week in Vienna, Olli J. Heinonen, head of safeguards at the international atomic agency, expressed concern about the documents to Mr. Schulte, diplomats said.

Scott Shane contributed reporting.

Correction: Nov. 7, 2006

A front-page article on Friday about concerns that certain Iraqi documents published in recent weeks by the federal government on a Web site set up in March might reveal nuclear secrets misstated the reason that Senator Pat Roberts, chairman of the Senate intelligence committee, gave for the campaign for the Web site. Mr. Roberts, Republican of Kansas, argued that the captured documents posted on the site would provide valuable information about Iraq under Saddam Hussein. He did not say that he believed they would support the idea that Mr. Hussein had resumed his unconventional arms programs before the 2003 invasion.