VIENNA — In the three years since the start of the Iran nuclear agreement, a cluster of buildings near the Austrian capital has served as an unblinking eye over Tehran’s most sensitive factories and research labs. But perhaps not for much longer.

Every day, workers arrive at the U.N. nuclear agency here to monitor live video from inside Iran’s once-secret uranium enrichment plants, part of an unbroken stream of data delivered by cameras and other remote sensors installed as part of the 2015 accord. Each week, scientists in lab coats analyze dust samples collected from across Iran, looking for minute particles that could reveal possible cheating.

Dispatchers track the movements of U.N. inspection teams that now work inside Iran every day of the year, checking and rechecking known nuclear facilities and occasionally venturing out to investigate tips about suspicious sites elsewhere.

The scrutiny carried out by officials of the International Atomic Energy Agency is a key component of the agreement, and it is unprecedented – not just for Iran, but for any country, anywhere in the world.

As the Trump administration announces that it is withdrawing from the pact, the U.N. watchdog agency is preparing for the possibility that its window into Iran’s nuclear affairs will abruptly slam shut.

President Trump announced on Tuesday that the United States will withdraw from the historic agreement, which was signed by the Obama administration as well as the leaders of Britain, France, Germany, Russia and China. While citing no evidence of major violations by Iran, Trump has repeatedly blasted the deal as a “disaster” while accusing Tehran of failing to live up to the spirit of the accord.

Trump’s animus toward the pact appeared to deepen last week after Israeli Prime Minister Benjamin Netanyahu made a dramatic television appearance to showcase evidence about nuclear weapons research conducted by Iran a decade before the agreement was signed. Trump asserted that the pact was useless because Tehran cannot be trusted to keep its word. “What we’ve learned has really shown that I’ve been 100 percent right,” Trump said.

The scrutiny carried out by officials of the International Atomic Energy Agency is a key component of the Iran nuclear agreement.

Yet by walking away from the deal, the Trump administration may lose its most important instrument for gauging whether Iran is telling the truth or not, according to former U.S. and U.N. officials and experts familiar with the IAEA’s oversight role. Many experts believe a collapse of the agreement will trigger a suspension of the unique, wide-ranging access accorded to the U.N. nuclear watchdog over the past three years.

In effect, by rejecting the deal as inadequate for preventing Iran from getting the bomb, Trump could make it harder for U.S. officials to detect a secret Iranian effort to build nuclear weapons, the former officials and experts said.

“We know more about Iran’s program with the deal than without it,” said former CIA director Michael V. Hayden, echoing an assessment voiced by current Director of National Intelligence Daniel Coats during congressional testimony earlier this year. Hayden, author a new book accusing the Trump White House of politicizing intelligence, said the Israeli revelations about Iran’s past nuclear research bolster the case for keeping the essence of the accord intact.

“The Iranians lie. They cheat,” Hayden said. “That’s why you need to have the best possible verification regime in place.”

Critics of the deal contend that its shortcomings outweigh the benefits of the IAEA’s intrusive oversight. Some argue that the agreement is inadequate for containing Iran’s long-term nuclear ambitions because several key restrictions are set to be phased out in 10 to 15 years. Others, including former officials of the watchdog group, fault the IAEA itself, saying the agency has not been sufficiently aggressive in demanding access to Iranian military facilities and fuller explanations about Iran’s past nuclear weapons research.

But U.N. officials say the pact’s transparency provisions have helped prevent war by replacing suspicions with hard facts. Yukiya Amano, the IAEA’s director general, told the agency’s 35-nation board of governors that Iran has complied so far with every request made by his inspectors. A collapse of the deal, he warned, would be “a great loss for nuclear verification.”

“The IAEA now has the world’s most robust verification regime in place in Iran,” the Japanese diplomat said in remarks after the board meeting in March. “As of today, I can state that Iran is implementing its nuclear-related commitments. It is essential that Iran continues to fully implement those commitments.”

As the world organization responsible for preventing the spread of nuclear weapons, the U.N.-affiliated IAEA has a long history with Iran, much of it troubled.

When Western intelligence agencies discovered that Iran was secretly building uranium enrichment plants – one at Natanz, in 2002, and another at an underground facility called Fordow in 2009 – the IAEA sent in its teams to investigate. In the years that followed, the agency confronted Iran repeatedly over what U.S. officials described as a clandestine nuclear-weapons research program that Iran apparently ended in 2003. Iran has consistently denied that it ever sought to acquire nuclear weapons and says its programs are directed toward energy production and medical research.

The IAEA was not a participant in the negotiations over the Iranian nuclear deal, but it has been an indispensable partner in its implementation. Since 2015, the agency’s inspectors have recorded and certified Iran’s compliance with each of several key components of the agreement. They confirmed, for example, that Iran had shipped out or eliminated 95 percent of its stockpile of enriched uranium, and dismantled or idled two-thirds of its centrifuge machines used in making nuclear fuel. Inspectors watched as Iran poured concrete into its partially completed nuclear reactor at Arak, bending to international concerns that the facility could become a future source of plutonium for nuclear bombs. They verified that Iran had halted uranium-enrichment activities at Fordow, the underground facility originally built inside a mountain as protection against airstrikes.

But the most demanding task for the agency’s inspection teams is the daily monitoring of Iran’s nuclear sites. Iran has for years allowed IAEA inspectors to visit its nuclear facilities and even granted permission for the installation of a few video cameras. But since 2015, the agency has enjoyed unparalleled access to every facet of Iran’s current nuclear program, from its uranium mines to the factory where it built its centrifuges.

The new oversight duties have meant an expanded IAEA presence in Iran itself. For the first time ever, the agency keeps a small cadre of inspectors inside Iran every day of the year, so it can handle the heavier workload and quickly respond to any reports about suspicious new sites.

IAEA inspectors have roamed through a total of 190 buildings around the country, while also making 60 “complementary access” calls – agency jargon for visits to facilities that are not part of Iran’s declared nuclear program.

Back at headquarters, specialists pore over terabytes of data collected by inspectors and transmitted to the Austrian capital over secure communications channels. In a large underground room beneath the IAEA’s office tower, banks of TV monitors flicker with live images from inside Iran’s sole functioning uranium enrichment plant. Computers keep tabs on the tamper-proof electronic seals placed by IAEA officials on more than 2,000 pieces of equipment, from storage bins to uranium-processing machines.

Each week, packages from Iran arrive at the IAEA’s laboratory complex in Seibersdorf, a village south of Vienna flanked by towering wind turbines and endless swaths of golden rapeseed. Some of the packets contain samples of uranium, which are tested to ensure that Iran is abiding by its promise to make only low-enriched fuel used in generating electricity, and not the highly enriched material that can produce a nuclear explosion.

Other parcels contain cloth swabs that inspectors carry with them when making their rounds. The swabs are used to scoop up dust from inside Iran’s nuclear facilities as well as from stair rails, window fixtures, vehicles and other random objects. Scientists in the Seibersdorf lab use million-dollar electronic microscopes and other sensors to scour the swabs for the tiniest traces of plutonium or highly enriched uranium that could point to a hidden weapons program.

Lab officials are not permitted to discuss their work publicly because of confidentiality agreements as well as the diplomatic sensitivity surrounding the Iran nuclear file. To ensure impartiality, the samples that arrive in Seibersdorf are stripped of identifying information, so the scientists never know the origins of the material they’re testing.

But, collectively, the IAEA’s oversight provides priceless, real-time information that can give U.S. officials confidence that Iran is honoring its commitments – or proof that they are not – said Ernest Moniz, a physicist and former energy secretary under the Obama administration who helped design the Iran’s deal’s verification mechanisms. Unlike the accord’s more ephemeral provisions, the IAEA’s expanded oversight role is permanent under the terms of the agreement, Moniz said.

“No other country has this kind of oversight. Iran has it forever,” Moniz said. “I don’t think this has been fully appreciated. The IAEA has increased its boots on the ground dramatically, and that’s being supplemented by advanced technology. They are collecting unbelievable amounts of data.”

Yet, even to ardent supporters of the agreement, last week’s revelations by Israel’s prime minister suggest that the IAEA has still more work to do.

In his televised speech from Tel Aviv, Netanyahu displayed thousands of captured documents and computer disks that he said contained a trove of details about “Project Amad,” Iran’s defunct weapons research program. The materials appear to show Iranian scientists conducting feasibility studies on the detonation of nuclear bombs and the mounting of warheads on Iran’s largest missiles.

Netanyahu said the records prove that Iran has consistently lied about its nuclear program when it signed the 2015 deal, and thus can’t be trusted to live up to its current agreements. The Israeli leader has argued that the pact should be either drastically changed – in part, to eliminate the agreement’s sunset provisions that would allow increased production of low-enriched uranium in the future – or completely scrapped.

U.S. and U.N. officials have known about Iran’s pre-2003 nuclear weapons program for more than a decade. In 2007, a major assessment by the U.S. intelligence community concluded that Iranian leaders had ordered the research, only to shut down the program in 2003 after the U.S. overthrow of Iran’s archrival, Iraqi dictator Saddam Hussein. Technical studies may have continued until as recently as 2009, U.S. officials have said.

Officials familiar with the Israeli revelations say the documents contain additional details about Iran’s weapons initiative, while again exposing Iran’s failure to come clean about its nuclear past.

“Iran has to explain it,” said Olli Heinonen, a former IAEA official who once led the agency’s oversight mission in Iran and confronted its leaders when the reports of secret nuclear research first came to light. “It looks to me that Iran was not absolutely forthcoming in addressing the concerns,” Heinonen said. “But there was political pressure to get the agreement implemented, so they went with the light touch.”

A critic of the Iran deal, the Finnish diplomat said he is troubled by Iran’s apparent decision to retain records from its illicit research. “A country that has a peaceful nuclear program doesn’t need to have this documentation,” said Heinonen, now a senior adviser on science and nonproliferation for the Foundation for Defense of Democracies, a Washington think tank.

Technically, Iran was not required to destroy its records under the 2015 deal. But proponents of the pact agree that Iran should be compelled to address the revelations about its nuclear past, in a formal proceeding, led by the watchdog agency that is best positioned to get answers: the IAEA.

“Real pressure needs to be put on the Iranians to explain the situation, and the IAEA has to be the point of the spear,” said Moniz, the former Energy secretary. “This is an opportunity to use the tools of the [nuclear agreement] to apply that pressure. Unfortunately, those tools could go away instantaneously if the president decides to walk away from the deal.”

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