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.  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."