WASHINGTON — Iranian nuclear engineer Mohsen Fakhrizadeh lectures weekly on physics at Tehran’s Imam Hossein University. Yet for more than a decade, according to documents attracting interest among Western governments, he also ran secret programs aimed at acquiring sensitive nuclear technology for his government.

Experts at the International Atomic Energy Agency have repeatedly invited Mr. Fakhrizadeh to tea and a chat about Iran’s nuclear work. But for two years, the government in Tehran has barred any contact with the scientist, who American officials say recently moved to a new lab in a heavily guarded compound also off-limits to U.N. inspectors.

The exact nature of his research — past and present — remains a mystery, as does the work of other key Iranian scientists whose names appear in documents detailing what U.N. officials say is a years-long, clandestine effort to expand the country’s nuclear capability. The documents, which were provided to the IAEA, the U.N. nuclear agency, in recent months by two countries other than America, partly match information in a stolen Iranian laptop turned over by Washington.

IAEA officials say these documents identify Mr. Fakhrizadeh and other civilian scientists as central figures in a secret nuclear research program that operated as recently as 2003. So far, however, Iran is refusing to shed light on their work or allow U.N. officials to question them. After being presented with copies of some of the new documents, Tehran denied that some of the scientists exist.

“When the allegations are raised, Iran simply dismisses them,” a Western diplomatic official familiar with the agency’s dealings with Iran said. “It insists that the documents are mostly fakes.”

The standoff over interview requests has cast a shadow over a five-year U.N. effort to excavate the truth about Iran’s nuclear past. In that search, Western anxieties have been compounded by Tehran’s reluctance to clarify the history of its interest in technologies that could be used for either nuclear power or weapons.

A similar set of uncertainties helped provoke the American war with Iraq, which the Bush administration justified partly by positing that Baghdad was deliberately concealing nuclear weapons research from U.N. inspectors. The outcome of that invasion suggests caution, however, since American troops were unable to find any convincing evidence of banned weapons work, and deposed Iraqi officials said they had been secretive to conceal from regional opponents that they had ended such work, not continued it.

In Iran’s case, U.N. officials say, the new evidence does not prove that the scientists carried out plans to build a nuclear device, but shows that Mr. Fakhrizadeh and other scientists struggled to master associated technologies. Several of the scientists, including Mr. Fakhrizadeh, appear to have moved freely between military and civilian research venues.

The documents purport to show advanced research into a variety of nuclear-related technologies, including uranium ore processing, warhead modification, and the precision-firing of high explosives of the type used to detonate a nuclear device. Other documents point to attempts by civilian scientists to purchase sensitive equipment of the kind Iran would eventually use in its uranium enrichment plants.