But some American intelligence officials winced when they heard Mr. Kerry’s “perfect knowledge” line, because they view their information as far less than perfect — and thus so is their knowledge about how close Iran came to design for a weapon. What may turn out to be far more important than what the agency learns, or does not learn, at Parchin is what it learns from documents and scientists who were part of Iran’s nuclear program.

Another confidential agreement between Iran and the international atomic agency lays out how inspectors will be able to resolve a list of 12 detailed questions about “possible military dimensions” of Iran’s past work, going far beyond Parchin. That agreement calls for resolving those questions by December, a herculean task since a serious investigation would require scores of interviews and reviewing complex documentation.

Iran has resisted that for the past four years. But Mr. Kerry told Congress that Iran would have to cooperate before sanctions were lifted. And it is not clear what “cooperation” means — whether the scientists simply have to talk to the inspectors, or actually come clean.

Olli Heinonen, the former chief inspector at the I.A.E.A., who led the agency’s team to Parchin in 2005, said last year: “You don’t need to see every nut and bolt, but you are taking a heck of a risk if you don’t establish a baseline of how far they went.”