It was after 10 p.m. on July 8, 2009, when Sandra Mansour answered her cellphone to the panicked voice of her daughter-in-law, Nasreen. A week earlier, Nasreen and her husband, Naji Mansour, had been detained in the southern Sudanese city of Juba by agents of the country's internal security bureau. In the days since, Sandra had been desperately trying to find out where the couple was being held. Now Nasreen was calling to say that she'd been released—driven straight to the airport and booked on a flight to her native Kenya—but Naji remained in custody. He was being held in a dark, squalid basement cell, with a bucket for a bathroom and a dense swarm of mosquitoes that attacked his body as he slept. "You have to get him out of there," Nasreen said. But she was unfamiliar with Juba and could only offer the barest details about where they'd been held. "He's in a blue building. You've seen it. It's not far from your hotel."

Sandra remembered passing a blue warehouse ringed by tall, razor-wire-topped fences. She hung up and turned to her daughter, Tahani, who'd flown to Juba to assist in tracking down her brother: "We've gotta go look for Naji." They packed food, water, and bug spray in case they found him. Then Sandra and Tahani laced up their sneakers, retrieved a flashlight, and slipped out onto a pitch-dark, deserted road.

Sudan's long-running civil war had ended a few years earlier, and Juba, once a malarial backwater on the White Nile, was poised to become the capital of the world's newest nation, South Sudan. The city had grown into a boomtown, its expansion fueled by newly discovered oil and an influx of foreign aid. Shacks and half-built concrete structures lined its maze of narrow, trash-strewn streets, and entrepreneurs rented out converted storage sheds for as much as $100 per night. Sandra, a US government contractor, lived in one of them.

The upstart city had a Wild West atmosphere. Rifle- and grenade-wielding bandits occasionally stormed poorly guarded compounds, and violent muggings and carjackings were commonplace. It was not safe to drive after dark, let alone walk, but Sandra and Tahani were desperate. "It was a very crazy thing to do," Sandra later recalled. "But it was the first lead we had, and there was nothing that was gonna stop us."

"He said, 'We want you to work with us. You have what it takes. You're the perfect candidate.' This is the shit you see in movies."

Sandra had grown up in Providence, Rhode Island; after leaving there on a backpacking trip in 1973, at age 21, she never stopped traveling. She later married a Sudanese economist, Ali Mansour, and together they lived and worked around the world, raising Naji and his three siblings to view their blue US passports as a ticket to a global life.

But that was before Naji landed in the crosshairs of the FBI and the family's comfortable expat existence started coming undone. For several months, Naji had been repeatedly interviewed by American authorities, detained and interrogated by Kenyan counterterrorism police, and ultimately forced into exile in Juba. Now he had vanished into a basement dungeon.

When Sandra and Tahani reached the blue warehouse, it appeared deserted. They circled it, then attempted to scale a back gate. When that failed, they shouted Naji's name into the lightless building.

Naji couldn't hear them. He was locked up about five miles away in another blue building, a Sudanese intelligence facility near a rocky outcropping called Witch Mountain. There, he was questioned repeatedly about whether he had ties to terrorism or Al Qaeda. The Sudanese interrogators threatened to kill him if he didn't tell them what he knew, and he could hear the screams of other prisoners being beaten.

Two weeks into his detention, Naji's jailers escorted him from his cell into a clean, bright room, where at last he saw a familiar face, a fellow American. It was an FBI agent he'd met with in the past. The agent told Naji that he could end his nightmare. "Help me help you," he said.

Naji first contacted me in April 2012, after I wrote a story about Yonas Fikre, an Oregon man who alleges that he was tortured in the United Arab Emirates after he refused to become an FBI informant. "I went through a similar ordeal," his email said.

Fikre's story fit a familiar pattern in which US citizens suspected of (often tangential) ties to terrorism were detained and questioned abroad by foreign security services—with evidence suggesting that American authorities orchestrated the detentions. This wasn't rendition, the controversial practice in which the CIA has shipped foreign nationals to allied countries where they were abused and tortured. Instead, American citizens were locked up abroad and interrogated by US agents in a manner that seemed designed to bypass their constitutional rights. Human rights advocates and civil libertarians have dubbed this practice "proxy detention."

The FBI acknowledges that foreign governments sometimes arrest Americans based on information the bureau provides. Here's how one FBI source explained it to me: If a guy the Saudi government suspected of terrorism traveled to the United States, we'd want to know. So it's only fair that we tip off the Saudis—or the Yemenis, Sudanese, or Egyptians—when an American suspected of terrorist ties enters their country.

What the bureau doesn't say is that since counterterrorism forces in many countries are funded and trained by the United States, the FBI's suggestions can sound a lot like orders—even when the suspects involved have never been charged with any crime.

"Often it has been US officials who do the real questioning, and sometimes the prisoners have been tortured and abused" by their foreign captors, says Hina Shamsi, an American Civil Liberties Union lawyer who works on the issue. "Unlawful detention and cruel treatment is wrong when the US government does it, and it's just as wrong if the United States asks another government to do it."

But as Naji discovered, the US government can do more than land you in prison overseas. It can reach deep into a family's life, upending relationships, destroying livelihoods, and leaving citizens stranded far from home without recourse or explanation.

NAJI WAS BORN IN San Diego in 1976, the first of four children, and his upbringing was a whirlwind tour of far-flung locales. As his father worked his way around the globe as an economic consultant to governments and businesses, Naji attended grade school on an American compound in Saudi Arabia and part of high school in a Crusades-era castle (complete with a moat) in Malta. When he was 20, the family visited Nairobi on a vacation, and they moved there for good later that year. All four Mansour children attended American colleges (Naji went to the University of Rhode Island for a year), and two of them now live in the United States. One of Naji's brothers joined the Marines and served two tours in Afghanistan.

Though he's spent most of his life abroad, Naji is an American by birth, by law, and by culture. He's a fan of the comedian Dave Chappelle, a rap aficionado—we had a lengthy conversation about the merits of Biggie Smalls versus Tupac Shakur—and a computer whiz.

Naji was raised Muslim, but he wasn't particularly religious as a child and didn't pray regularly until he was 18. He was more interested in pan-Africanism—in Naji's words, the idea "that a united Africa could be independent from foreign intervention, and people's lives would improve"—a popular school of thought among his professors at the United States International University in Nairobi. But in 1998, Naji dropped out of school and married a young Ugandan named Shamila. They moved to England and had a daughter, but without legal residency he couldn't find work beyond odd jobs, and Shamila eventually returned to Uganda and gave birth to their son. Naji moved back to Nairobi in April 2000 in hopes that Sandra, then working as a housing and travel coordinator for USAID (she supervised luggage handling during then-first lady Laura Bush's May 2007 visit), could set him up with a job at the US Embassy. In 2002, Naji and Shamila divorced. Soon after, he married Nasreen and they started a family together in Nairobi.

Sandra Mansour shops for sneakers with her graddaughters in Nairobi. Until December, she'd been blocked from entering Kenya for two years.

Naji spent 2008 working for a tech company in Dubai. During his stay, he occasionally invited friends from work and the mosque he attended to bunk at his mom's house—which the family dubbed "Hotel Sandra"—if they ever visited Nairobi. A week after returning home from Dubai, he got a call from a guy named Muhammad whom he barely remembered. "I got a couple of friends. Could you put them up?" Muhammad asked. "They're coming this week." Naji agreed, expecting more details, but Muhammad abruptly hung up.

A few days later, Naji's phone rang again. The men had arrived. "I was like, 'Uh…okay.'" He hopped into the family's old Mercedes—a memento of his father, who died in 2006—and drove to pick them up.

The two visitors, Bilal el-Berjawi and Mohamed Gamal Sakr, both 24, said they were grad students who had traveled to Kenya to study the farmers who grow miraa (also known as khat), a mild amphetamine that's legal there. But they seemed to do little other than hang around. They watched the Mansours' seven tortoises trudge around the backyard. They prayed. After a week, Naji gently inquired when Berjawi and Sakr would be moving on. They told him they were waiting for their families to wire some money to continue their travels and research.

Around 2 p.m. on February 23, 2009, a bit more than two weeks into the pair's visit, dozens of armed men surrounded the house. Naji and Nasreen were out. The family's maid, Violet Mugasiali, was home with their young daughters. "All of a sudden the bell started ringing nonstop," Mugasiali remembered. The men said they were with Kenya's counterterrorism police, a special American-funded unit. She called Naji and Nasreen as the police burst into the compound, arresting Berjawi and Sakr and confiscating computers belonging to Naji.

Nasreen rushed home while Naji contacted Al-Amin Kimathi, a prominent lawyer and the head of Kenya's Muslim Human Rights Forum. Naji had volunteered for Kimathi's organization, where he helped to investigate the detention and rendition of Muslims who had been arrested in Kenya as they fled Ethiopia's US-backed invasion of neighboring Somalia. Some of the detainees were militants affiliated with the Islamic Courts Union, which spawned al-Shabaab, the terrorist group that pledged fealty to Al Qaeda in 2012. But many were simply refugees attempting to escape the violence.

Now Naji called on Kimathi for a favor. Since he didn't have a Kenyan passport, he was worried he might be deported; Kimathi helped him obtain an official document saying he could stay. Then Naji turned himself in for interrogation. (Nasreen was also arrested; she was held for 30 hours before being released without charges.)

The Kenyan police told Naji that Berjawi and Sakr had been plotting a terrorist attack—perhaps targeting the Westgate, Nairobi's fanciest mall. (The two men were deported to the United Kingdom, where they had grown up, but soon returned to Africa. In 2010, the UK revoked their citizenship for alleged ties to terrorism, and both were later killed by US drone strikes in Somalia—where, the British government said, they had joined al-Shabaab.)

Naji was questioned about his ties to the would-be terrorists, whom he told his interrogators he barely knew. They also confronted him with terrorism-related files that were discovered on his computers. Some were mundane, such as research papers and think tank studies about Islamic extremism, but others were more suspicious, like martyrdom videos and al-Shabaab propaganda. Naji said that he was no terrorist, but was fascinated by the causes of terrorism and curious about how the religious doctrine of jihad was used to justify it; in his spare time, he spent hours doing online research.

"I'm telling you, you might get hit by a car. That is not a threat. That is a solid piece of advice. But you don't want to take it."

Following two days of questioning, the Kenyan authorities let Naji go. Sandra suggested that he reach out to the US Embassy, where she had many contacts, to report what had happened and clear up any suspicions about his ties to Berjawi and Sakr. She connected him with a diplomatic security officer named Michael Fogarty. When they later met at the embassy, Fogarty asked Naji if he would "consider speaking to some law enforcement." Then he brought in a heavyset, blondish man he introduced as Jeffrey Roberts, the embassy's deputy regional security officer. Roberts, in turn, ducked out and came back with two FBI agents. The shorter, dark-haired one introduced himself as Mike Jones. (This is a pseudonym. The FBI told Mother Jones that, because of the agent's role in the "recruitment of sources" overseas for counterterrorism work, revealing his identity would put him at risk.)

Naji recounted the story of how Berjawi and Sakr had come to stay with him, but the agents asked few questions about them. Instead, Jones grilled Naji about Kimathi, his acquaintance from the Muslim Human Rights Forum, and inquired about one of Nasreen's distant cousins, a man named Omar Awadh Omar. (Omar is currently being prosecuted in Uganda for helping orchestrate the 2010 bombings in Kampala that targeted soccer fans watching the World Cup finals.) Jones asked Naji whether he had ever brought "guns, money, or people for violence" to Somalia or other countries. Naji said no. After about an hour, the conversation wrapped up; Naji agreed to sit down with Roberts again later that week.

Roberts showed up to their next meeting "in his shades, looking like Top Gun," Naji recalled. And he had a proposition. "He said, 'We want you to work with us. You have what it takes. You're the perfect candidate.' I asked him, 'What exactly are you talking about?' It was very surreal. This is the shit you see in movies. I was laughing."

But Roberts wasn't joking. "He said, 'We can give you rewards for information, or we can put you on full time. But that would require a continuous flow of information.'" Naji understood that his houseguests had placed him under a "cloud of suspicion," he later told me. But Roberts didn't seem interested in that anymore. "Mostly, it was 'We need your help,'" Naji said.

It's not hard to see why the US government would view Naji as an ideal informant. He is religious, conservative, and speaks English and Arabic. He's calm under pressure. He had crisscrossed the globe as a volunteer escort for refugees being resettled through the International Organization for Migration. When he traveled, he went to mosques and counted on the hospitality of strangers to find a bed for the night, and through this he had made connections with dozens of other religious Muslim men around the world.

This is precisely the kind of community that the FBI is trying to track and infiltrate. The bureau's network of paid informants has expanded rapidly since 9/11, and now includes more than 15,000, rivaling the scale of the J. Edgar Hoover era. A guy like Naji—an expatriate working in countries where terrorists operate—would be a real catch.

But to someone not facing criminal charges, the FBI doesn't have much to offer by way of enticement. "The problem for many American Muslims who have been approached by the FBI to become informants is that they aren't involved in criminal conspiracies and don't have relationships with criminals," says Mike German, an ex-FBI agent who now works for the Brennan Center for Justice at New York University. "Instead, they are being asked to spy broadly against their religious community. That creates a conundrum because the person may be perfectly willing to help the FBI fight terrorism but simply has no information to provide."

Naji told Roberts he was planning to move to Juba, where his mother had taken a job with a company called Management Systems International that did work for USAID. He hoped to launch a business there selling rugged laptops. "We could use you there, too," Roberts said.

"No, man," Naji replied. Spying was "not something I want to play around with," especially in a country like Sudan. "This is Africa. There's no law for me here."