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 United Nations 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 considers 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 has said he will announce Tuesday whether 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.

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 of 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.

Read the full story at The Washington Post.

Scientists at the International Atomic Energy Agency in Seiberdorf, Austria, review results from tests of nuclear material collected abroad. The agency is helping ensure Iran’s compliance with the 2015 nuclear accord that put restrictions on Iran’s nuclear program. (Joby Warrick/The Washington Post)

Israeli Prime Minister Benjamin Netanyahu describes how Iran has continued with its nuclear capabilities during a presentation at the Israeli Defense Ministry in Tel Aviv on April 30. (Jim Hollander/EPA-EFE/Shutterstock)