The CIA agent tapped softly on the hotel room door. After the keynote speeches, panel discussions and dinner, the conference attendees had retired for the night. Audio and visual surveillance of the room showed that the nuclear scientist's minders from the Islamic Revolutionary Guard Corps were sleeping but he was still awake. Sure enough, he opened the door, alone.

According to a person familiar with this encounter, which took place about a decade ago, the agency had been preparing it for months. Through a business front, it had funded and staged the conference at an unsuspecting foreign institution of scientific research, invited speakers and guests, and planted operatives among the kitchen workers and other staff, just so it could entice the nuclear expert out of Iran, separate him for a few minutes from his guards, and pitch him one-on-one. A last-minute snag had almost derailed the plans: The target switched hotels because the conference's preferred hotel cost $75 more than his superiors in Iran were willing to spend.

To show his sincerity and goodwill, the agent put his hand over his heart. "Salam habibi," he said. "I'm from the CIA, and I want you to board a plane with me to the United States." The agent could read the Iranian's reactions on his face: a mix of shock, fear and curiosity. From prior experience with defectors, he knew the thousand questions flooding the scientist's mind: What about my family? How will you protect me? Where will I live? How will I support myself? How do I get a visa? Do I have time to pack? What happens if I say no?

The scientist started to ask one, but the agent interrupted him. "First, get the ice bucket," he said.

"Why?"

"If any of your guards wake up, you can tell them you're going to get some ice."

In perhaps its most audacious and elaborate incursion into academia, the CIA secretly spent millions of dollars staging scientific conferences around the world. Its purpose was to lure Iranian nuclear scientists out of their homeland and into an accessible setting where its intelligence officers could approach them individually and press them to defect. In other words, the agency sought to delay Iran's development of nuclear weapons by exploiting academia's internationalism, and pulling off a mass deception on the institutions that hosted the conferences and the professors who attended and spoke at them. The people attending the conference had no idea they were acting in a drama that simulated reality but was stage-managed from afar. Whether the national security mission justified this manipulation of the professoriate can be debated, but there's little doubt that most academics would have balked at being dupes in a CIA scheme.

More than any other academic venue, conferences lend themselves to espionage. Assisted by globalization, these social and intellectual rituals have become ubiquitous. Like stops on the world golf or tennis circuits, they sprout up wherever the climate is favorable, and draw a jet-setting crowd. What they lack in prize money, they make up for in prestige. Although researchers chat electronically all the time, virtual meetings are no substitute for getting together with peers, networking for jobs, checking out the latest gadgets, and delivering papers that will later be published in volumes of conference proceedings. "The attraction of the conference circuit," English novelist David Lodge wrote in "Small World," his 1984 send-up of academic life, is that "it's a way of converting work into play, combining professionalism with tourism, and all at someone else's expense. Write a paper and see the world!"

The importance of a conference may be measured not only by the number of Nobel Prize winners or Oxford dons it attracts, but by the number of spies. U.S. and foreign intelligence officers flock to conferences for the same reason that Army recruiters concentrate on low-income neighborhoods: They make the best hunting grounds. While a university campus may have only one or two professors of interest to an intelligence service, the right conference — on drone technology, perhaps, or ISIS — may have dozens.

"Every intelligence service in the world works conferences, sponsors conferences, and looks for ways to get people to conferences," says a former CIA operative.

"Recruitment is a long process of seduction," said Mark Galeotti, senior researcher at the Institute of International Relations Prague and former special adviser to the British foreign office. "The first stage is to arrange to be at the same workshop as a target. Even if you just exchange banalities, the next time you can say, 'Did I see you in Istanbul?'"

The FBI warned American academics in 2011 to be cautious about conferences, citing this scenario: "A researcher receives an unsolicited invitation to submit a paper for an international conference. She submits a paper and it is accepted. At the conference, the hosts ask for a copy of her presentation. The hosts hook a thumb drive to her laptop, and unbeknownst to her, download every file and data source from her computer."

The FBI and CIA swarm conferences, too. At gatherings in the United States, says a former FBI agent, "foreign intelligence officers try to collect Americans; we try to collect them." The CIA is involved with conferences in various ways: It sends officers to them; it hosts them through fronts in the Washington area, so that the intelligence community can tap academic wisdom; and it mounts sham conferences to reach potential defectors from hostile countries.

The CIA monitors upcoming conferences worldwide and identifies those of interest. Suppose there is an international conference in Pakistan on centrifuge technology: The CIA would send its own agent undercover, or enlist a professor who might be going anyway to report back. If it learns that an Iranian nuclear scientist attended the conference, it might peg him for possible recruitment at the next year's meeting.

Intelligence from academic conferences can shape policy. It helped persuade the George W. Bush administration — mistakenly, as it turned out — that Saddam Hussein was still developing weapons of mass destruction in Iraq. "What our spies and informants were noticing, of course, was that Iraqi scientists specializing in chemistry, biology, and, to a lesser extent, nuclear power kept showing up at international symposia," former CIA counterterrorism officer John Kiriakou wrote in a 2009 memoir. "They presented papers, listened to the presentation of others, took copious notes, and returned to Jordan, where they could transmit overland back to Iraq."

Some of those spies may have drawn the wrong conclusions because they lacked advanced degrees in chemistry, biology or nuclear power. Without expertise, agents may misunderstand the subject matter, or be exposed as frauds. At conferences hosted by the International Atomic Energy Agency in Vienna on topics such as isotope hydrology and fusion energy, "there's probably more intelligence officers roaming the hallways than actual scientists," says Gene Coyle, who worked for the CIA from 1976 to 2006. "There's one slight problem. If you're going to send a CIA guy to attend one of these conferences, he has to talk the talk. It's hard to send a history major. 'Yes, I have a PhD in plasma physics.' Also, that's a very small world. If you say you're from the Fermi Institute in Chicago, they say, 'You must know Bob, Fred, Susie.'"

Instead, Coyle says, the agency may enlist a suitable professor through the National Resources Division, its clandestine domestic service, which has a "working relationship" with a number of scientists. "If they see a conference in Vienna, they might say, 'Professor Smith, that would seem natural for you to attend.'"

"Smith might say, 'I am attending it, I'll let you know who I chatted with. If I bump into an Iranian, I won't run in the opposite direction.' If he says, 'I'd love to attend, but the travel budget at the university is pretty tight,' the CIA or FBI might say, 'Well, you know, we might be able to take care of your ticket, in economy class.'"

A spy's courtship of a professor often begins with a seemingly random encounter — known in the trade as a "bump" — at an academic conference. One former CIA operative overseas explained to me how it works. I'll call him "R."

"I recruited a ton of people at conferences," R told me. "I was good at it, and it's not that hard."

Between assignments, he would peruse a list of upcoming conferences, pick one, and identify a scientist of interest who seemed likely to attend after having spoken at least twice at the same event in previous years. R would assign trainees at the CIA and NSA to develop a profile of the target — educational background, college instructors and so on. Then he would cable headquarters, asking for travel funding. The trick was to make the cable persuasive enough to score the expense money, but not so compelling that other agents who read it, and were based closer to the conference, would try to preempt him.

Next he developed his cover — typically, as a businessman. He invented a company name, used GoDaddy.com to build a website and printed business cards. He created billing, phone and credit card records for the nonexistent company. For his name, he chose one of his seven aliases.

R was no scientist. He couldn't drop in a line about the Riemann hypothesis as an icebreaker. Instead, figuring that most scientists are socially awkward introverts, he would sidle up to the target at the edge of the conference's get-together session and say, "Do you hate crowds as much as I do?" Then he would walk away.

"The bump is fleeting," R says. "You just register your face in their mind."

No one else should notice the bump. It's a rookie mistake to approach a target in front of other people who might be minders assigned by the professor's own country. The minders would report the conversation, compromising the target's security and making them unwilling or unable to entertain further overtures.

For the rest of the conference, R would "run around like crazy," bumping into the scientist at every opportunity. With each contact, called "time on target" in CIA jargon and counted in his job performance metrics, he insinuated himself into the professor's affections. For instance, having done his homework, R would say he had read a wonderful article on such-and-such topic but couldn't remember the author's name. "That was me," the scientist would say, blushing.

After a couple of days, R would invite the scientist to lunch or dinner and make his pitch: His company was interested in the scientist's work and would like to support it. "Every academic I have ever met is constantly trying to figure how to get grants to continue his research. That's all they talk about." They would agree on a specific project, and the price, which varied by the scientist's country: "One thousand to five thousand dollars for a Pakistani. Korea is more." Once the CIA pays foreign professors who are unaware at first of the funding source, it controls them, because exposure of the relationship might imperil their careers or even their lives in their native country.

Scientific conferences have become such a draw for intelligence agents that one of the biggest concerns for CIA operatives is interference from agency colleagues trapping the same academic prey. "We tend to flood events like these," a former CIA officer who writes under the pseudonym Ishmael Jones observed in his 2008 book, "The Human Factor: Inside the CIA's Dysfunctional Intelligence Culture." At one 2005 conference in Paris that he anticipated would be a "perfect watering hole for visiting rogue state weapons scientists," Jones recalled, his heart sank as he glanced across the room and saw two CIA agents (who were themselves professors). He avoided their line of sight while he roamed the gathering, eyeballing nametags and trawling for "people who might make good sources," ideally from North Korea, Iran, Libya, Russia or China.

"I'm surprised there's so much open intelligence presence at these conferences," Karsten Geier said. "There are so many people running around from so many acronyms." Geier, head of cybersecurity policy for the German foreign office, and I were chatting at the Sixth Annual International Conference on Cyber Engagement, held in April 2016 at Georgetown University in Washington, D.C. The religious art, stained-glass windows and classical quotations lining Gaston Hall enveloped the directors of the NSA and the FBI like an elaborate disguise as they gave keynote addresses on combating one of the most daunting challenges of the twenty-first century: cyberattacks.

The NSA's former top codebreaker spoke, as did the ex-chairman of the National Intelligence Council, the deputy director of Italy's security department and the director of a center that does classified research for Swedish intelligence. The name tags that almost all of the seven hundred attendees wore showed that they worked for the US government, foreign embassies, intelligence contractors, or vendors of cyber-related products, or they taught at universities.

Perhaps not all of the intelligence presence was open. Officially, 40 nations — from Brazil to Mauritius, Serbia to Sri Lanka — were represented at the conference, but not Russia. Yet, hovering in the rear of the balcony, a slender young man, carrying a briefcase, listened to the panels. No name tag adorned his lapel. I approached him, introduced myself and asked his name. "Alexander," he said, and, after a pause, "Belousov."

"How do you like the conference?"

"No," he said, trying to ward off further inquiries. "I am from Russian embassy. I don't have any opinions. I would like to know, that's all."

I proffered a business card, and requested his, in vain. "I am here only a month. My cards are still being produced."

I persisted, asking about his job at the embassy. (A subsequent check of a diplomatic directory showed him as a "second secretary.") He looked at his watch. "I am sorry. I must go."

When the CIA wants Professor John Booth's opinion, it phones him to find out if he's available to speak at a conference. But the agency's name is nowhere to be found on the conference's formal invitation and agenda, which invariably list a Beltway contractor as the sponsor.

By hiding its role, the CIA makes it easier for scholars to share their insights at its conferences. They take credit for their presentations on their curriculum vitae without disclosing that they consulted for the CIA, which might alienate some academic colleagues as well as the countries where they conduct their research.

An emeritus professor of political science at the University of North Texas, Booth specializes in studying Latin America, a region where history has taught officials to be wary of the CIA. "If you were intending to return to Latin America, it was very important that your CV not reflect" these kinds of presentations, Booth told me in March 2016. "When you go to one of these conferences, if there are intelligence or defense agency principals there, it's invisible on your CV. It provides a fig leaf for participants. There's still some bias in academia against this. I don't go around in Latin American studies meetings saying I spent time at a conference run by the CIA."

The CIA arranges conferences on foreign policy issues so that its analysts, who are often immersed in classified details, can learn from scholars who understand the big picture and are familiar with publicly available sources. Participating professors are generally paid a $1000 honorarium, plus expenses. With scholarly presentations followed by questions and answers, the sessions are like those at any academic meeting, except that many attendees — presumably, CIA analysts — wear name tags with only their first names.

Of ten intelligence agency conferences that Booth attended over the years, most recently a 2015 session about a wave of Central American refugee children pouring into the United States, the CIA and Office of the Director of National Intelligence directly ran only one or two. The rest were outsourced to Centra Technology Inc., the leader of a growing industry of Beltway intermediaries — "cutouts," in espionage parlance — that run conferences for the CIA.

The CIA supplies Centra with funding and a list of people to invite, who gather in Centra's Conference Center in Arlington, Virginia. It's "an ideal setting for our clients' conferences, meetings, games, and collaborative activities," according to Centra's website.

"If you know anything, when you see Centra, you know it's likely to be CIA or ODNI [Office of the Director of National Intelligence]," said Robert Jervis, a Columbia University professor of international politics and longtime CIA consultant. "They do feel that for some academics thin cover is useful."

Established in 1997, Centra has received more than $200 million in government contracts, including $40 million from the CIA for administrative support, such as compiling and redacting classified cables and documents for the five-year Senate Intelligence Committee study of the agency's torture program. As of 2015, its executive ranks teemed with former intelligence officials. Founder and chief executive Harold Rosenbaum was a science and technology adviser to the CIA. Senior vice president Rick Bogusky headed the Korea division at the Defense Intelligence Agency. Vice president for research James Harris managed analytic programs at the CIA for 22 years. Peggy Lyons, director of global access, was a longtime CIA manager and officer with several tours in East Asia. David Kanin, Centra analytic director, spent 31 years as a CIA analyst.

Like Booth, Indiana University political scientist Sumit Ganguly has spoken at several Centra conferences. "Anybody who works with Centra knows they're in effect working for the U.S. government," he said. "If it said CIA, there are others who would fret about it. I make no bones about it to my colleagues. If it sticks in their craw, it's their tough luck. I am an American citizen. I feel I should proffer the best possible advice to my government."

Another political scientist, who has given four presentations for Centra, said he was told that it represented unnamed "clients." He didn't realize the clients were US intelligence agencies until he noticed audience members with first-name-only name tags. He later ran into one or two of the same people at an academic conference. They weren't wearing name tags and weren't listed in the program.

Centra strives to mask its CIA connections. It removed its executives' biographies from its website in 2015. The "featured customers" listed there include the Department of Homeland Security, the FBI, the Army and 16 other branches of the federal government — but not the CIA. When I phoned Rosenbaum and asked him about Centra holding conferences for the CIA, he said, "You're calling the wrong person. We have nothing to do with that." And then he hung up.

I dropped by Centra's offices on the fifth floor of a building in Burlington, Massachusetts, a northern suburb of Boston. The sign-in sheet asked visitors for their citizenship and "type of visit": classified or not. The receptionist fetched human resources director Dianne Colpitts. She politely heard me out, checked with Rosenbaum and told me that Centra wouldn't comment.

"To be frank," she said, "our customers prefer us not to talk to the media."

For Iranian academics escaping to the West, academic conferences are a modern-day underground railroad. The CIA has taken full advantage of this vulnerability. Beginning under President George W. Bush, the U.S. government had "endless money" for covert efforts to delay Iran's development of nuclear weapons, the Institute for Science and International Security's David Albright told me. One program was the CIA's Operation Brain Drain, which sought to spur top Iranian nuclear scientists to defect.

Because it was hard to approach the scientists in Iran, the CIA enticed them to academic conferences in friendly or neutral countries, a former intelligence officer familiar with the operation told me. In consultation with Israel, the agency would choose a prospect. Then it would set up a conference at a prestigious scientific institute through a cutout, typically a businessman, who would underwrite the symposium with $500,000 to $2 million in agency funds. The businessman might own a technology company, or the agency might create a shell company for him, so that his support would seem legitimate to the institute, which was unaware of the CIA's hand. "The more clueless the academics are, the safer it is for everybody," the ex-officer told me. Each cutout knew he was helping the CIA, but he didn't know why, and the agency would use him only once.

The conference would focus on an aspect of nuclear physics that had civilian applications and also dovetailed with the Iranian target's research interests. Typically, Iran's nuclear scientists also held university appointments. Like professors anywhere, they enjoyed a junket. Iran's government sometimes allowed them to go to conferences, though under guard, to keep up with the latest research and meet suppliers of cutting-edge technology — and for propaganda.

"From the Iranian point of view, they would clearly have an interest to send scientists to conferences about peaceful uses of nuclear power," Ronen Bergman told me. A prominent Israeli journalist, Bergman is the author of "The Secret War with Iran: The 30-Year Clandestine Struggle Against the World's Most Dangerous Terrorist Power," and is working on a history of Israel's central intelligence service, the Mossad. "They say, yes, we send our scientists to conferences to use civilian technology for a civilian purpose."

The CIA officer assigned to the case might pose as a student, a technical consultant or an exhibitor with a booth. His first job would be to peel the guards away from the scientist. In one instance, kitchen staff recruited by the CIA poisoned the guards' meal, leaving them incapacitated by diarrhea and vomiting. The hope was that they would attribute their illness to airplane food or an unfamiliar cuisine.

With luck, the officer would catch the scientist alone for a few minutes, and pitch him. He would have boned up on the Iranian by reading files and courting "access agents" close to him. That way, if the scientist expressed doubt that he was really dealing with the CIA, the officer could respond that he knew everything about him, even the most intimate details — and prove it. One officer told a potential defector, "I know you had testicular cancer and you lost your left nut."

Even after the scientist agreed to defect, he might reconsider and run away. "You're constantly re-recruiting the guy." Once he was safely in a car to the airport, the CIA coordinated the necessary visas and flight documents with allied intelligence agencies. It would also spare no effort to bring his wife and children to the United States — though not his mistress, as one scientist requested. The agency would resettle the scientist and his family and provide long-term benefits, including paying for the children's college and graduate school.

Enough scientists defected to the United States, through academic conferences and other routes, to hinder Iran's nuclear weapons program, the ex-officer familiar with the operation told me. He said an engineer who assembled centrifuges for Iran's nuclear program agreed to defect on one condition: that he pursue a doctorate at MIT. Unfortunately, the CIA had spirited him out of Iran without credentials such as diplomas and transcripts. At first, MIT refused the CIA's request to consider him. But the agency persisted, and the renowned engineering school agreed to accommodate the CIA by waiving its usual screening procedures. It mustered a group of professors from related departments to grill the defector. He aced the oral exam, was admitted and earned his doctorate.

MIT administrators denied any knowledge of the episode. "I'm completely ignorant of this," said Gang Chen, chairman of mechanical engineering.

However, two academics corroborated key elements of the story. Muhammad Sahimi, a professor of petroleum engineering at the University of Southern California who studies Iranian nuclear and political development, told me that a defector from Iran's nuclear program received a doctorate from MIT in mechanical engineering.

Timothy Gutowski, an MIT professor of mechanical engineering, said, "I do know of a young man that was here in our lab. Somehow I learned that he did work on centrifuges in Iran. I started thinking, 'What went on here?'"

With Iran's agreement in 2015 to limit nuclear weapons development in return for lifting of international sanctions, recruitment of defectors from the program by U.S. intelligence lost some urgency. But if President Donald Trump scraps or seeks to renegotiate the deal, which he denounced in a September speech to the United Nations General Assembly, CIA-staged conferences to snag key Iranian nuclear scientists could make a clandestine comeback.