Iran's nuclear ambitions - and its honesty - are being tested by fresh IAEA findings of undeclared centrifuge designs, components, and past experiments that could be linked to a weapons program.
Iran denies that it is pursuing nuclear weapons. It has accepted snap inspections by the UN International Atomic Energy Agency (IAEA), and signed the Additional Protocol of the Nuclear Non-Proliferation Treaty (NPT) in December.
But a series of inspections since then has left "discrepancies and unanswered questions," the Director General Mohamed ElBaradei told the IAEA Board of Governors in a confidential report circulated on Tuesday.
"The people who are studying this are getting more and more suspicious," says a Western diplomat in Vienna. Late last year Iran said its declaration was "full and complete, and this raises the question: 'Is this everything?' "
The IAEA has been granted unimpeded access to all civilian and military sites in Iran, the report notes. And on Tuesday - just hours before the report was released and leaked to the press - Iran offered to expand the suspension of its uranium enrichment program.
"Iran was telling the truth all the time ... Inspectors are here to prove this," says Hussein Shariatmadari, a representative of Iran's Supreme Leader, and head of the conservative Kayhan publishing group in Tehran. "The Americans are looking for an excuse to drag our case to the UN Security Council." Any gaps between Iran's declarations and recent IAEA findings are "accidental," says Mr. Shariatmadari.
"There was a lack of coordination between the Foreign Ministry, National Security Council, and the Atomic Energy Ministry," says Abbas Maleki, a former deputy foreign minister who now heads a Caspian Studies institute in Tehran. "Iran wants to show that it is honest, and that it doesn't want to do something against the Tehran Declaration [the watershed nuclear deal Iran signed with Britain, France, and Germany last October]. That is the best way for Iran."
But the IAEA still has questions, especially about how much Iran benefited from a pipeline of black market nuclear technology that originated in Pakistan. Libya received bomb plans from Pakistan, and pursued a nuclear program that the IAEA report says used "very similar" technology "from the same foreign sources" as Iran.
As a signatory to the NPT, Iran has always claimed its right to nuclear technology for power production. US officials have just as strongly alleged that Iran was using its civilian program as a cover to make nuclear weapons.
Revelations from last year about four separate uranium enrichment processes in Iran - which can be used to produce nuclear fuel for energy, or for bombs - sparked US-led IAEA pressure on the Islamic republic to open fully to inspections.
"[Iran's leaders] are trying to become incrementally more transparent, while testing the trust of the US and the EU," says Mohammad Hadi Semati, a Tehran University political scientist, who is now at the Carnegie Endowment for International Peace in Washington. "If they can talk honestly with the IAEA, credibility could be restored. But if this pattern continues, it will be a big problem."
"Deep down there is still a consensus, a fundamental commitment [in Tehran] to no nuclear weapons," Mr. Semati says. But there has also been a domestic backlash, which may explain tough rhetoric about limiting the suspension of uranium enrichment programs, and Iran bridling at IAEA rules.
"They did not anticipate the political reaction from forces inside Iran, that say they sold out the regime," says Semati. "So they had to look tougher."
But if Tehran is trying to appease a domestic constituency, the price may be a loss of credibility in the international community.
"A substantial fraction of European observers confused a big move on the part of Iran [last fall] with a big enough move," says Michael Levi, a nuclear nonproliferation expert at the Brookings Institution in Washington. "In theory, this should be the final demonstration that Iran is acting in bad faith. Iran is confronted with a tough decision. If they turn over more documents, it shows they are more open. But it also proves they did something wrong."
Tough IAEA scrutiny last year led the agency to report that Iran had been hiding a vast clandestine nuclear effort for 18 years. Little direct evidence points to a weapons program, though some experiments point to weapons uses: Iran dabbled in uranium metal; and tests more than a decade ago with polonium-210 are described for the first time by the IAEA in this report.
While polonium can be used for "nuclear batteries," the latest report says, it can also be used, with beryllium, as a "neutron initiator in some designs of nuclear weapons."
Also detailed is Iran's failure to declare design plans for a more advanced centrifuge known as a P-2 - and components manufactured for mechanical testing. The IAEA called the omission "a matter of serious concern" that "runs counter to Iran's declaration."
Inspectors also found two different types of uranium contamination at two separate sites in Iran - raising questions about Iranian claims that the traces came solely from imported equipment.
In one room at the Kalaye Electric Company in Tehran, IAEA sampling found uranium enriched to levels 30 times as high as Iran has declared it has achieved using centrifuge technology. Those levels, the report said, "suggests the presence of more than just trace quantities of such material."
"The hole in which Iran was in has stopped shrinking - it got deeper," says a Western diplomat in Tehran, about the discrepancies. "It's not clear if it was an oversight or a deliberate deception, but chances are it's the latter. It doesn't look good for Iran.
"We're worried when bits of information tumble out of the closet," the diplomat says. "It doesn't necessarily mean 'weapons program' when a P-2 turns up. The question is whether there is any link between P-2 components, and the ever- present possibility there could be other facilities."
Iran denies that any more undisclosed facilities exist, and senior officials have been meeting with the IAEA in Vienna this week to take the edge off the new discoveries.
On Tuesday, Iran promised to broaden its temporary suspension of enrichment activities, by halting the assembly of new centrifuges and making new components. Any production under existing contracts will be put under IAEA seal.
"It's very significant, because they can't do anything. Their centrifuges can't spin; uranium can't be enriched," says the Vienna diplomat. "What's clear is that the IAEA is on to them - they probably underestimated the ability of the inspectors."