As the C.I.A.’s assessment was making its way through the government, late this summer, current and former military officers and consultants told me, a new element suddenly emerged: intelligence from Israeli spies operating inside Iran claimed that Iran has developed and tested a trigger device for a nuclear bomb. The provenance and significance of the human intelligence, or HUMINT, are controversial. “The problem is that no one can verify it,” the former senior intelligence official told me. “We don’t know who the Israeli source is. The briefing says the Iranians are testing trigger mechanisms”—simulating a zero-yield nuclear explosion without any weapons-grade materials—“but there are no diagrams, no significant facts. Where is the test site? How often have they done it? How big is the warhead—a breadbox or a refrigerator? They don’t have that.” And yet, he said, the report was being used by White House hawks within the Administration to “prove the White House’s theory that the Iranians are on track. And tests leave no radioactive track, which is why we can’t find it.” Still, he said, “The agency is standing its ground.”

The Pentagon consultant, however, told me that he and other intelligence professionals believe that the Israeli intelligence should be taken more seriously. “We live in an era when national technical intelligence”—data from satellites and on-the-ground sensors—“will not get us what we need. HUMINT may not be hard evidence by that standard, but very often it’s the best intelligence we can get.” He added, with obvious exasperation, that within the intelligence community “we’re going to be fighting over the quality of the information for the next year.” One reason for the dispute, he said, was that the White House had asked to see the “raw”—the original, unanalyzed and unvetted—Israeli intelligence. Such “stovepiping” of intelligence had led to faulty conclusions about nonexistent weapons of mass destruction during the buildup to the 2003 Iraq war. “Many Presidents in the past have done the same thing,” the consultant said, “but intelligence professionals are always aghast when Presidents ask for stuff in the raw. They see it as asking a second grader to read ‘Ulysses.’ ”

HUMINT can be difficult to assess. Some of the most politically significant—and most inaccurate—intelligence about Iraq’s alleged weapons of mass destruction came from an operative, known as Curveball, who was initially supplied to the C.I.A. by German intelligence. But the Pentagon consultant insisted that, in this case, “the Israeli intelligence is apparently very strong.” He said that the information about the trigger device had been buttressed by another form of highly classified data, known as MASINT, for “measuring and signature” intelligence. The Defense Intelligence Agency is the central processing and dissemination point for such intelligence, which includes radar, radio, nuclear, and electro-optical data. The consultant said that the MASINT indicated activities that “are not consistent with the programs” Iran has declared to the I.A.E.A. “The intelligence suggests far greater sophistication and more advanced development,” the consultant said. “The indications don’t make sense, unless they’re farther along in some aspects of their nuclear-weapons program than we know.”

In early 2004, John Bolton, who was then the Under-Secretary of State for Arms Control (he is now the United Nations Ambassador), privately conveyed to the I.A.E.A. suspicions that Iran was conducting research into the intricately timed detonation of conventional explosives needed to trigger a nuclear warhead at Parchin, a sensitive facility twenty miles southeast of Tehran that serves as the center of Iran’s Defense Industries Organization. A wide array of chemical munitions and fuels, as well as advanced antitank and ground-to-air missiles, are manufactured there, and satellite imagery appeared to show a bunker suitable for testing very large explosions.

A senior diplomat in Vienna told me that, in response to the allegations, I.A.E.A. inspectors went to Parchin in November of 2005, after months of negotiation. An inspection team was allowed to single out a specific site at the base, and then was granted access to a few buildings there. “We found no evidence of nuclear materials,” the diplomat said. The inspectors looked hard at an underground explosive-testing pit that, he said, “resembled what South Africa had when it developed its nuclear weapons,” three decades ago. The pit could have been used for the kind of kinetic research needed to test a nuclear trigger. But, like so many military facilities with dual-use potential, “it also could be used for other things,” such as testing fuel for rockets, which routinely takes place at Parchin. “The Iranians have demonstrated that they can enrich uranium,” the diplomat added, “and trigger tests without nuclear yield can be done. But it’s a very sophisticated process—it’s also known as hydrodynamic testing—and only countries with suitably advanced nuclear testing facilities as well as the necessary scientific expertise can do it. I’d be very skeptical that Iran could do it.”

Earlier this month, the allegations about Parchin reëmerged when Yediot Ahronot, Israel’s largest newspaper, reported that recent satellite imagery showed new “massive construction” at Parchin, suggesting an expansion of underground tunnels and chambers. The newspaper sharply criticized the I.A.E.A.’s inspection process and its director, Dr. Mohamed ElBaradei, for his insistence on “using very neutral wording for his findings and his conclusions.”