U.N. watchdog findings that Iran may have worked on designing an atomic bomb helped the West tighten sanctions on Tehran which ultimately forced it, U.S. officials say, to enter serious talks on reining in its nuclear program.
But as Iran and six world powers enter a crucial stage in their efforts to hammer out a long-term deal by late July, the issue of how to handle suspicions that the Islamic Republic has carried out nuclear weapon research could emerge as a serious stumbling block.
At issue is whether to press Iran to fully admit to any such activity in the past - even if it complicates the search for a broader deal to end the nuclear dispute - or whether to focus more on ensuring that whatever happened then has since stopped.
The outcome could have far-reaching consequences for the standing of the International Atomic Energy Agency, which has long struggled to end Iranian stonewalling of its investigations and which says clarification of all its questions is vital.
Israel, Iran's arch-foe that is itself believed to have such weapons, and hawks in the U.S. Congress may pounce on any final accord if they feel it is not sufficiently robust in requiring Tehran to address the entirety of the IAEA's suspicions.
However, Iran's denials of nuclear arms aspirations - especially a religious decree by Supreme Leader Ayatollah Ali Khamenei that ruled out acquiring and using nuclear weapons because this was banned by Islamic values - could make it virtually impossible for it to own up to any illicit work.
It is unrealistic to expect Iran to confess to "everything they know and did", said Jim Walsh, a research associate at the Massachusetts Institute of Technology (MIT), adding that he doubted a full public "mea culpa" by the country was needed.
Robert Einhorn, a former top U.S. State Department official once involved in talks with Tehran, said: "While full and honest Iranian disclosure of past activities is undoubtedly the best result, such an outcome faces formidable obstacles."
A Western diplomat critical of Iran - not from any of the countries involved in the diplomacy - put it more bluntly, saying Iran had problems in addressing the allegations because the suspicions were "true and they are going to get caught".
Iran says the accusations are false and baseless but has offered to work with the IAEA to clear them up.
The negotiations between Iran and the major powers that got under way in 2013 and paved the way for a thaw after years of hostility have so far concentrated on preventing Iran from obtaining the fissile material needed for a future bomb, rather than on any past effort to acquire nuclear weapon technology.
"PERSUASIVE" IAEA FINDINGS ON IRAN
U.S. officials stress, however, that the Islamic Republic must resolve IAEA concerns, suggesting that some sanctions relief could hinge on it, without spelling out exactly how this should be done in practice.
"We have been very clear that one cannot achieve a (final agreement) without Iran making progress and resolving with the IAEA the concerns about the possible military dimensions of the program," a senior U.S. official said on Tuesday.
"The pace of that resolution will have an impact on ... the relief that Iran is seeking," the official said ahead of this week's fourth round of talks in Vienna between Iran and the United States, France, Russia, China, Germany and Britain.
Another Western official said nobody expects the IAEA's inquiry to be completed by the self-imposed July 20 deadline for a diplomatic deal but Iran needed to show a "meaningful change in engagement" with the U.N. agency's inquiry by that time.
In a sign of the slow progress so far, a three-hour meeting between Iran and the IAEA on Monday ended without an agreement on new action to tackle its requests for access to sites, information and officials it says it needs for its inquiry.
On the one such issue that Iran has so far agreed to provide information - concerning the development of detonators that can be used, among other things, to set off a nuclear device - diplomatic sources said the IAEA wanted further clarifications.
The detonator issue was part of a landmark IAEA report in November 2011 containing a trove of intelligence indicating past activity in Iran which could be applied to developing nuclear weapons, some of which it said might still be continuing.
The Vienna-based U.N. agency said its report was based on intelligence from more than 10 countries - believed to include the United States and Israel - as well as on its own sleuthing, and that the information was credible overall. The IAEA says it has since received new information backing up its analysis.
The allegations under IAEA scrutiny range from explosives testing to research on what experts describe as an atomic bomb trigger. The IAEA also wants Iran to address suspicions of computer modeling to calculate nuclear explosive yields, and preparatory experiments that could be useful for atomic tests.
Einhorn said that there was a "persuasive body of information" indicating that Iran, at least until 2003, pursued a program to develop a nuclear weapons capability.
"Resolution of these long-outstanding questions is essential for the agency to certify that all nuclear material in Iran is in peaceful use," ex-IAEA chief inspector Olli Heinonen said.
NO "BOX-CHECKING EXERCISE"
But two other former IAEA officials questioned its case.
The 2011 report is "mostly a rehash of many years of very old information. The analysis of that information is amateurish and flawed," Robert Kelley, a former IAEA director involved in dismantling Iraq's nuclear weapons program in the 1990s, said.
Nevertheless, he said Iran should come clean about its past activities, which he said strongly suggested it had a nuclear weapons exploration program, "if nothing more than a response to the program they knew was under way in Iraq" in 1991.
Another former senior IAEA official, Tariq Rauf, took issue with the authentication of documents the IAEA has received from outside the agency and that form the basis for its suspicions.
"It is impossible for the agency to prove the veracity of the source of the information and the information itself," Rauf,
at the Stockholm International Peace Research Institute, said.
David Albright of the Institute for Science and International Security think-tank said an accord that does not cover Iran addressing the IAEA's concerns would undermine the agreement's credibility and that of the U.S. administration.
The powers, in the interest of concluding a deal, might urge the IAEA to accept "what it would consider a less than satisfactory demonstration" by Iran that the suspicions are unfounded, said Mark Hibbs of the Carnegie Endowment think-tank.
Einhorn suggested, however, that the matter could be tackled without a full disclosure by Iran. "Such an admission is not necessary in order to develop a sufficient degree of confidence that those activities are not continuing."
Editing by Mark Heinrich