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.