Iran ridicules the material as fake, maintaining that the trove is full of forged documents created by the Central Intelligence Agency or Israel’s Mossad. (The atomic agency’s chief, Yukiya Amano, dismissed that allegation in an interview last summer, saying the inspectors had confirmed the documents by consulting other sources.)

The problem is that the documents, if real, would undercut Iran’s argument that its nuclear ambitions are entirely peaceful, centering on the production of radioisotopes for medicine and electrical power for economic growth.

Expertise in warhead design, as opposed to atomic fuel production, is far more ephemeral and hard to track. It can also be less ambiguous. Some nuclear parts have application only to making weapons, such as neutron spark plugs at the core of some atom bombs. In contrast, uranium can fuel both nuclear arms and reactors that make electricity — it can light cities or annihilate them.