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In his small Silicon Valley office, Salem Khan covered his face with his hands and sobbed deeply.

He was crying about his 24-year-old son, Jaffrey, a troubled young man who had struggled in school and in life. Khan long feared his child would one day do something terrible — and now he had.

He had joined ISIS.

With his wife and 18-year-old brother-in-law in tow, Jaffrey left behind the moneyed, manicured precincts of the Bay Area and offered his services to one of the most savage terrorist organizations in the world.

The father tried to suggest his son was impressionable, a lost soul, an easy mark for bloodthirsty fanatics selling the fantasy of an Islamic caliphate.

"He was gullible," Salem Khan said. "You know the people looking to scam you out of money or something? They're looking for people like him."

Khan lives in a $2 million house and runs a medical marketing company in Palo Alto, in the shadow of elite Stanford University — 7,000 miles and a world away from where this story began, in southern Turkey.

That's where, in March, we met a man who claimed to be an ISIS defector. He called himself Abu Mohammed and gave NBC News a thumb drive that he said he stole from an ISIS commander and smuggled out in a baby's diaper.

Related: Richard Engel Explains How NBC News Got ISIS Files

It contained the personnel files of thousands of foreign fighters who joined ISIS in 2013 and 2014, including names, some home addresses, emergency contacts and whether they wanted to be suicide bombers. Experts at the West Point Combating Terrorism Center and other counterterrorism specialists verified the files are authentic.

Richard Engel and Abu Mohammed NBC News

Through the documents, NBC News has identified at least 15 U.S. citizens or residents who went overseas and joined ISIS. Some were already known as recruits; other names have never been made public before.

Over the course of two months, NBC News pieced together the story of a single cell with three members.

Jaffrey Khan and Rasel Raihan enlisted with ISIS on July 11, 2014, the documents show. Though she is not in the files, they were joined by Zakia Nasrin, now 24, who is Rasel's older sister and Jaffrey's wife — and who is now the mother of a 10-month-old baby girl, according to relatives.

Related: What Leaked Files Reveal About ISIS Recruits

Their story stretches from the middle-class suburbs of Ohio and the silicon jewel of California to the Syrian capital of the so-called "Islamic State."

It's the story of a high-school valedictorian who wanted to be a doctor, of a studious computer gamer who had Harvard in his sights, and of a pot-smoking hip-hop fan who traded in atheism for extremism.

And it's the story of two immigrant families who built new lives in America only to have the heirs to their dream join an organization hell-bent on destroying it.

Related: 15 Who Left the United States to Join ISIS

The Khan family said in a statement that Jaffrey's actions have been "heartbreaking for our family and we do not support his personal choices."

Rasel and Zakia's father, Mohammad Mannan, told NBC News, he feels he doesn't have children any longer.

"They are lost," he said.

The Misfit

"He was always a problem kid," Salem Khan said of Jaffrey, his first-born son.

As a one-year-old, the boy had trouble walking, and his difficulties seemed only to grow from there. He was diagnosed with attention-deficit disorder and took Ritalin, his father said. "He could never concentrate for more than 10 minutes on anything."

School was an ordeal, marred by disruptive behavior and even physical incidents, a source familiar with his disciplinary record told NBC News. Educational records show Jaffrey attended two different middle schools then shuttled between Gunn High School and an alternative program, sometimes transferring mid-year.

His father said he ended up in a group home at one point, and didn't continue onto college after leaving high school in 2010.

In the affluent, competitive landscape of Palo Alto, Jaffrey just "never fit in" like his popular younger sister, one Gunn classmate said.

"He always struck me as a slightly disturbed person, sort of bullying and disruptive. Not in a class clown way, but that there was something wrong with him," said another school acquaintance, Nate Levine.

Jaffrey Khan NBC News

Jaffrey's parents, Pakistani immigrants, divorced when he was young, and he was mainly raised by his father, who remarried and had two more children. His mother, who also remarried and had four more children, lives 90 minutes northeast of Palo Alto. During weekend visits with her, Jaffrey grew close to a cousin, Ahmed Khan.

As teens, they played basketball together and stayed up late watching movies. Jaffrey was a jokester who was into rap music, smoked pot and could spend hours on the Internet, Ahmed said.

The cousin thought Jaffrey was a "good kid" but unmoored. Ahmed felt the Muslim faith could ground Jaffrey, but he showed no interest.

"He wasn't religious at all," Ahmed said. "He was an atheist, actually, at that time." According to his father, Jaffrey had dabbled in Mormonism and Hinduism —and his full-throttle embrace of a radical form of Islam came as a shock to the family.

It was his mother's husband who got him to convert in earnest, Ahmed said. Jaffrey, who often seemed to go overboard with new interests, underwent a rapid transformation in the months after leaving high school.

"He became really religious. He grew his beard out," Ahmed said. "He started sleeping on the floor. He said, 'I'm not gonna use a bed no more because our prophet Mohammed, didn't use a bed.' He would eat with his hands and not use forks and spoons."

He began doing online research into conflicts in the Muslim world "and that's when he started changing," Ahmed said. "He was getting angry…He would be really aggressive."

Ahmed believes Jaffrey was "brainwashed" by his Internet contacts. His attitude became "more hateful" toward Americans, he said.

"He'd say that we live in a country with non-believers. And he said we're surrounded by a bunch of sinful people and we should move to a Muslim country," Ahmed recalled. "[He was] always talking about fighting and 'We have to kill these non-Muslim people, they're raping our women and killing our people.' Those kind of things."

As his fanaticism deepened, Jaffrey grew more distant from his Americanized relatives, they said. But he would not be alone for long. Through a Muslim dating service, he was searching for a wife.

The Over-Achiever

In a 2009 local news article about a student farm project, Zakia Nasrin spoke about her plans for a career in medicine — and her hope of making the world a better place.

"I'm really interested in sustainable agriculture because as a doctor, I'd be focused on third-world countries and this could really apply," she said.

At the time of the interview, she was a senior at the selective and rigorous Metro Early College High School in Columbus, where classmates remembered her as intelligent and thoughtful, fully capable of handling a curriculum that included university classes.

"She was probably one of the smartest and most functioning people in the school, and that was a pretty high standard," Zach Brazik told NBC News.

Zakia Nasrin Adam Cairns / This Week via The Columbus Dispatch

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Zakia and her younger brother, Rasel, had moved to Ohio from Bangladesh in 2000. The family eventually bought a $162,000 house in Reynoldsburg, a middle-class community just outside Columbus.

Their hard-working parents — the father works two jobs, including nights at a gas station — placed a premium on academic achievement. And Zakia delivered.

At Metro, she was one of just four students chosen to take part in the innovative student farm program at Ohio State. A video shows her working with the crops in a T-shirt and leggings, talking in a soft voice about the marvel of a harvest.

She graduated from Metro with a perfect 4.0 average and was named one of 26 valedictorians in the Class of 2010. After a cancer-research internship that summer, she was supposed to start at Ohio State.

Instead, she seemed to drop out of sight. Her best friend from high school, Meagan Jones, became so worried when Zakia suddenly cut off contact that she reached out to her parents in late 2010. Jones was bewildered to learn that Zakia had left Ohio for California and married Jaffrey Khan.

The Gamer

Throughout childhood, Rasel Raihan had seen his big sister Zakia as his mentor — "guiding me right from wrong," as he once wrote a friend. When she moved to the West Coast in 2010, just as he was about to start high school, he felt abandoned, he wrote.

Following in her footsteps, he enrolled at Metro, where he fell in with a small crew of self-described "nerds" whose idea of delinquency was slacking off on homework to play computer fantasy games.

"He was a genius — really, really smart," said one of his pals, Sam Knisely.

"He was basically the student that everyone should aspire to be," said another friend, Phil Chu. "He wanted to be a doctor. He wanted to help people."

This undated photo shows Rasel Raihan. via Facebook

When he wasn't hitting the books and getting straight A's, Rasel was playing computer games. For a while, the crew's favorite was a role-playing anime world called Elsword Online. Rasel chose a sword-fighter as his avatar.

"He definitely wanted to be the cool, mysterious character," Chu laughed.

Rasel figured out a way to game the game — and the success he and his friends had after that did not go unnoticed by the moderators, who curtly informed them they had been caught cheating and banned them.

Chu remembers it well because it was around that time in 2013 that Rasel, then a junior, began to withdraw socially. "He just started talking less and less," Chu said. "He stopped appearing online. He stopped seeing me after school, seeing any of us."

Rasel would later tell Chu in an online chat that he was rocked by getting a B in physics and overwhelmed by friends who leaned on him for academic help.

"Maybe the dark nature of anime and manga had something to do with it, but I gave up all hope on humans," he would explain. "All they care about is themselves. And I'm working to become a doctor and save these ungrateful scum? Forget it."

By his senior year, he was "a shell of the old Rasel," Knisely said. Chu, who was already in college, heard that Rasel — once so proud of a letter from Harvard inviting him to apply — was cutting classes.

That fall, Rasel failed his chemistry midterm and was plunged into a deep depression, he would tell Chu. "I was at my all-time low," he wrote. "I seriously contemplated suicide." But, he added, "I decided it wouldn't be fair for me to 'run away' and have others clean up my mess."

He stopped going to school and started sleeping 12 hours a night. Things boiled over during finals week, when he finally "came clean" about his emotional issues.

He was referred to a psychiatrist who put him on the antidepressant Lexapro, he would later tell Chu. But he "wasn't satisfied" with this solution and decided to seek out answers from someone else: his sister Zakia.

The Marriage

By the time of Rasel's crisis in 2013, his sister had been married to Jaffrey Khan for three years. While she didn't seem particularly religious to those who knew her at Metro, there was no question about her post-matrimonial zeal.

Zakia Nasrin at graduation Adam Cairns / This Week via The Columbus Dispatch

In high school, Zakia didn't cover her head. But after her marriage, she wore a full face veil and Jaffrey kept her away from all men, even relatives.

"He never showed her," Ahmed Khan recalled. "When he would come over, she'd stay in a car or she'd stay in the living room and he'd make all the men of the family go in the room so we wouldn't see her. So I didn't know how she looked, anything about her."

The couple eventually returned to Ohio "to be close to Zakia's parents," Jaffrey's father, Salem, said.

He said he didn't approve of their fundamentalist lifestyle, but he supported them financially, allowing both to work remotely for his company. An online business profile for the firm lists Zakia as an employee in 2012.

Zakia was enrolled at Ohio State in the fall of 2012 as a junior, university records show. She and Jaffrey rented an apartment in a small complex on Riverview Ave. in Columbus, according to a lease obtained by NBC News.

In January 2013, Meagan Jones, Zakia's high school friend, got a Facebook message from her out of the blue. She replied, but bizarrely, the next message from the same account was written by Jaffrey.

"Zakia got married. I'm her husband lol," it said.

"Anyway, Zakia talks about you a lot and misses you as you were her best friend, so I told her to contact you, but she's too shy/embarrassed. So I convinced her to at least send you a message on facebook, and she agreed on the condition that she doesn't have to look at your reply or anything lol."

Jones wrote back saying she wanted to keep in touch, but she never heard another word.

Missed Opportunities?

Of all the places in Columbus they could have lived, Zakia and Jaffrey picked one with a connection to Islamic extremism.

In 2007, the Riverview Ave. building was home to Christopher Paul, an American who later pleaded guilty to training al Qaeda bomb-makers to attack targets in Europe and the United States. Although Paul is serving 20 years in prison, his wife still lives in the building. She told NBC News that she didn't know Jaffrey and Zakia, even though they had lived in the unit right next to her for nearly two years.

Christopher Paul is currently serving 20 years in prison for training al Qaeda bomb-makers. Franklin County Sheriff's Office via AP

According to the landlord, some of the apartment building's residents worshipped at the nearby Omar Ibn El-Khattab mosque, which was linked in years past to militants. The head of the mosque's board said Jaffrey quietly attended for only a few weeks.

"He pretty much kept to himself, minded his own business, came, attended prayers," Basil Mohamed Gohar said. "He didn't speak about anything radical. He didn't talk about anything strange."

Elsewhere, though, he had raised red flags.

In September 2012, an acquaintance heard through friends that Jaffrey had traveled overseas and might be mixed up with jihadists. The tipster, who asked not to be named, contacted the FBI through its website and provided NBC News with a record of the report.

The FBI asked some follow-up questions, but how it handled the tip is unclear. It's unknown whether Jaffrey had actually left the country at the time of the tip or what country he was said to have visited.

Sources and relatives told NBC News that Jaffrey and Zakia made a trip to Kenya, but the date is unclear.

According to the cousin, Ahmed, Zakia was pregnant at the time and planned to have the baby there, but she miscarried and they returned to the United States after a couple of months. Her brother Rasel told Chu that his sister and brother-in-law reported losing their passports while abroad and "caused a big fuss at the embassy asking to come back to America and were on the watch list for terrorism."

In the Skype chat, Rasel said he learned about it the summer of 2013 when the FBI showed up at the family home in Ohio. Agents grilled him and confiscated his computer, becoming alarmed by emails about "urban terror" that were actually just a reference to a computer game the old crew played, he wrote.

The Ohio family hired a lawyer to deal with the FBI, Rasel wrote. He said he had little to tell law enforcement anyway, because he and his sister had been "estranged" since she got married.

The Disappearance

The depression and academic problems that Rasel confronted in the fall of 2013, during his senior year of high school, brought about a rapprochement with his sister, he said.

"I broke down in front of her and asked her a simple question," he wrote in the Skype chat with Chu in the fall of 2014.

"For what reason was I born?"

Her answer: "You were created to worship your creator."

Rasel told Chu it was like "the dark clouds had parted and I could see the light." He said he began practicing Islam, found his depression "cured," and stopped taking his medicine.

Now that he was 18, Rasel left home and moved in with Jaffrey and Zakia — who were getting ready to make a big move themselves.

In May 2014, the three of them packed up the Riverview Ave. apartment and headed for California. Their Ohio landlord, Jim Ryan, said he mailed them a deposit check, minus $90 because they had changed the locks without permission, but it was never cashed.

The ISIS documents obtained by NBC News indicate that two months after moving out of Ohio, Jaffrey and Rasel entered Syria at Tel Abyad, a city near the Turkish border that had just been captured by the so-called caliphate.

A member loyal to ISIS waves an flag in Raqqa June 29, 2014. Stringer / Reuters / REUTERS

Jaffrey took the fighter name Abu Ibrahim al-Amriki (the American); Rasel would be called Abu Abduallah al-Amriki.

Salem Khan said he heard from his son only twice after he left the U.S., once when he was in Turkey and once after he crossed into Syria. "I yelled at him," the father said of their last conversation in September 2014.

Rasel resurfaced in November 2014 in the Skype chat with Chu, who thought he had carried through on a plan to return to Bangladesh. He said he was overseas, but was vague about where.

"It's a long and messy story," Rasel wrote.

Jaffrey and Rasel arrived in Syria as the U.S. was ramping up its air campaign against ISIS. According to members of the Khan family and to a senior U.S. intelligence source, Rasel was killed in Syria, though the timing and circumstances are murky.

Zakia — the girl who dreamed of being a doctor — ended up working with her husband in a hospital in Raqqa, ISIS' main hub in Syria, according to Salem Khan.

The couple is in sporadic contact with Jaffrey's mom, Shaista Khan, who said she hears from them every couple of months through Telegram, an encrypted text messaging application.

Sometimes the news is disturbing. Ahmed Khan said Shaista called his house once to report that Jaffrey had observed the mass beheading of Christians in Syria.

In recent months, the pair sent a photo of their daughter, Miriam. Posted to Shaista's Facebook page, it shows a sleeping baby swaddled in a fuzzy yellow onesie, oblivious to the atrocities being carried out by the organization her parents joined. Experts say ISIS encourages its fighters to marry and raise families, to give their so-called state a sense of stability.

Jaffrey Khan and Zakia Nasrin had a baby girl, Miriam, after leaving the U.S., according to relatives. Khan's mother posted this photo on her Facebook page. via Facebook

Jaffrey, Rasel and Zakia are among just a few dozen Americans who have managed to get to Syria and sign up with ISIS. FBI Director James Comey said last week that the number of Americans seeking to join its ranks overseas — already low on a per capita basis — had fallen even further to just one a month since August.

In an interview with NBC News, Assistant Attorney General John Carlin, the Justice Department's point person on national security, said he could not discuss specific cases or say whether federal investigators had missed opportunities to stop the trio from physically joining ISIS.

"We have a responsibility to stop those who would travel overseas to join the Islamic State and the Levant, both because of what they do over there — murder, rape, slavery," Carlin told NBC News.

"But also because we don't want them going over there, getting additional skills, training, and coming back as hardened operatives to do complex attacks here in the United States."

Law enforcement has "gotten a lot better at disrupting those who would go join as foreign terrorist fighters" Carlin said, but stressed that civilians who see radicalization in their midst have a responsibility, too.

Carlin cited an analysis he said shows that in four out of five terrorist cases, someone in the community is aware of what's happening — but only half of them alert the authorities.

"It's so important that if a family knows that someone has traveled, that they communicate and go into law enforcement," Carlin said. "You don't want your loved one being killed overseas. You don't want your loved one killing other people overseas."

U.S. Assistant Attorney General for National Security John Carlin Alex Wong / Getty Images, file

There is no indication anyone from the Ohio or California families raised the alarm, and Ahmed Khan said they were too scared or never thought it would go as far as it did.

All of the parents said they have spoken to the FBI since their children left the country. They expressed fear of retribution and a desire to put a dark chapter behind them and move on with their lives in America.

Salem Khan said that while he knew his son had problems, he felt that once he was an adult, there was only so much he could do to influence him. On the other side of the country, Mannan suggested that it was American permissiveness that had allowed Zakia and Rasel to embrace anti-U.S. radicalism, saying if the family had stayed in Bangladesh, they would still be together.

"There I have the right to control my children and I can control my children. Here I don't have any rights when my children have 18 years," he said.

Mannan said he had no idea what happened to his children after they left the U.S.

"I don't have any communication with them and I don't want to have anything to do with them," he said. "I don't even want to see their faces."