Two years ago, American intelligence agencies published a detailed report concluding that Tehran halted its efforts to design a nuclear weapon in 2003. But in recent months, Britain has joined France, Germany and Israel in disputing that conclusion, saying the work has been resumed.

A senior American official said last week that the United States was now re-evaluating its 2007 conclusions.

The atomic agency’s report also presents evidence that beyond improving upon bomb-making information gathered from rogue nuclear experts around the world, Iran has done extensive research and testing on how to fashion the components of a weapon. It does not say how far that work has progressed…

American officials say that in the direct negotiations with Iran that began last week, it will be vital to get the country to open all of its suspected sites to international inspectors. That is a long list, topped by the underground nuclear enrichment center under construction near Qum that was revealed 10 days ago.