By ELAINE SCIOLINO Published: October 9, 2008

PARIS – International nuclear inspectors are investigating whether a Russian scientist helped Iran conduct complex experiments on how to detonate a nuclear weapon, according to European and American officials. As part of the investigation, inspectors at the International Atomic Energy Agency are seeking information from the scientist, who they believe acted on his own as an adviser on experiments described in a lengthy document obtained by the agency, the officials said.

The officials, who spoke on the condition of anonymity because the investigation is under way, said that the document appeared authentic, without explaining why, but they made it clear that they did not think the scientist was working on behalf of the Russian government.

Still, it is the first time that the nuclear agency has suggested that Iran may have received help from a foreign weapons scientist in developing nuclear arms.

The American and European officials said the new document, written in Farsi, was part of an accumulation of evidence that Iran had worked toward developing a nuclear weapon, despite Iran’s claims that its atomic work over the past two decades has been aimed solely at producing electrical power.

In February, in a closed-door briefing at the agency’s headquarters in Vienna, its chief nuclear inspector presented diplomats from dozens of countries with newly declassified evidence – documents, sketches and even a video – that he said raised questions about whether Iran had tried to design a weapon.

Among the data presented by Olli Heinonen, the chief inspector, were indications that the Iranians had worked on exploding detonators that are critical for the firing of most nuclear weapons.

When the Iranian envoy at the briefing called the charges “groundless” and protested that the tests were for conventional arms, Mr. Heinonen replied that the experiments were “not consistent with any application other than the development of a nuclear weapon,” two participants said. He called the shape and timing involved in the firing systems and detonators “key components of nuclear weapons.”

At the same time, Mr. Heinonen acknowledged that the agency “did not have sufficient information at this stage to conclude whether the allegations are groundless or the data fabricated.”

The new document under investigation offers further evidence of such experiments, the Western officials said.

Iranian officials have said repeatedly that the documents the agency is using in its investigation of Iran’s past nuclear activities are fabrications or forgeries, and that any experiments were not related to nuclear weapons.

Iran has said the same about the new evidence, although the agency has not shown the full document to government officials in Tehran. Instead, Iran has been given only five pages of excerpts that have been translated from Farsi into English.

The Western officials said that the conditions under which the inspectors obtained the document prohibited them from revealing it in full to the Iranians, out of fear that doing so could expose the source of the document.

These restrictions present a problem for Mohamed ElBaradei, the agency’s director general, who is pressing Iran to reveal its past nuclear activity. “I cannot accuse a person without providing him or her with the evidence,” he said last year.

Although officials would not say how they had obtained the new document, it was first publicly mentioned in an agency report in May as one of 18 documents presented to Iran in connection with suspected nuclear weapons studies. At the time it was described as a “five-page document in English” about experiments with a complex initiation system to detonate a large amount of high explosives and to monitor the detonation with probes. There was no indication that the document was a translation of a much longer, more comprehensive document in Farsi.

The original, Farsi document is described by officials familiar with it as a detailed narrative of experiments aimed at creating a perfectly timed implosion of nuclear material.

According to experts, the most difficult challenges in developing nuclear weapons are creating the bomb fuel and figuring out how to compress and detonate it.

That was followed by an agency report last month that revealed that Iran might have received “foreign expertise” in its detonator experiments.

A senior official with links to the agency said then that a foreign government was not involved. He ruled out the involvement of Libya and the remnants of the network run by Abdul Qadeer Khan, the Pakistani metallurgist who built the world’s largest black-market sales operation for nuclear technology. But he would not comment further.

European and American officials now say that the “foreign expertise” was a reference to the Russian scientist, but they offered only scant details. They said that the scientist was believed to have helped guide Iranians in the experiments, but that he did not write the document.

Nor is he thought to have been affiliated with the civilian electric power plant that is being rebuilt by Russia at the Iranian port of Bushehr, and which Russia has agreed to fuel with nuclear material, the officials said.

Russia says it opposes any effort by Iran to obtain a weapon, but cooperation by Russian companies and individuals with some aspects of Iran’s nuclear program dates back years.

In the late 1990s, Russia’s scientific and technical elite, reeling from the collapse of the Soviet Union, forged ties to Iran, which paid hard currency for aid in weapons and technical programs. Western experts say the help extended to Tehran’s atomic efforts, but there was never any proof in those years of a Russian link to nuclear weapons development.

“The Iranians were very active in recruiting and paying Russian scientists to provide them with assistance in their nuclear program,” said Gary S. Samore, a National Security Council official during the Clinton administration who now directs studies at the Council on Foreign Relations.

He said he had no recollection of Russian aid in the design of Iranian nuclear arms but added that it could have happened. “It’s plausible to me that they at some point paid a Russian nuclear expert to provide assistance,” he said in an interview.

Asked about the potential contribution of the Russian scientist in detonator experimentation, a senior Russian official who has long followed Iran’s nuclear program said, “It is difficult for me to add anything.”

William J. Broad contributed reporting from New York, and David E. Sanger from