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A New York City man who admits he betrayed his country by joining ISIS has come out of the shadows to tell publicly for the first time how he was seduced by the terrorist organization, why he fled after five months and how he's helping the FBI.

In an exclusive interview with NBC News, the 27-year-old describes how his life took a dark turn while he was a student at elite Columbia University.

In a course called Muslims in Diaspora, he watched the controversial Dutch film "Submission," which depicts a woman in a sheer burqa with passages from the Koran written on her nude body. The man, who is being identified only as Mo, told NBC News that he found the video "really humiliating" and he turned to the Internet for answers to his questions about Islam.

"I started compromising my American side for my Islamic side," Mo said.

It was the beginning of a journey that would lead him into the heart of ISIS territory in Syria — and then back to the United States, where he is in federal custody, facing up to 25 years in prison, and cooperating with authorities.

Prosecutors allowed Mo to speak to NBC News for the first-ever U.S. television interview with an American ISIS defector.

They believe his story of being seduced by ISIS' promise of a utopian Islamic state only to find fear and violence — including severed head on spikes — might deter others from signing up.

"I've let my family down. I've let my nation down and I've let God down and I have a lot to make up for," Mo said.

He said he grew up in a Muslim household but his family wasn't particularly strict. He played baseball and ran track and focused on education. After high school, he attended a community college, where he made the dean's list, then dropped out after his sister's death.

After a stint selling cellphones, he got into Columbia, but abandoned the Ivy League campus after the video he saw in religion class. He began driving a taxi and spending a lot of time doing "research" on Islam and conflicts in the Muslim world online.

ISIS, which had not yet begun to release videos of beheaded hostages, appealed to him with its vision of a pure Islamic state without geographic boundaries, and he began thinking of going abroad, he said.

Mo's online activities had caught the attention of the FBI and agents showed up to question him — and warn that he shouldn't travel to Syria.

But Mo didn't listen. Instead, he bought a ticket to Turkey, packed his bags and wrote a letter to his parents. "I was planning not to come back and it was a farewell," he said.

With his life savings of a few thousand dollars in his pocket, he flew to Istanbul in June 2014, caught a connecting flight to Gaziantep and a bus to Urfa, which was close to Raqqa, the Syrian city that would become ISIS' capital.

"I was kind of winging it," Mo said.

Once settled in a hotel in Urfa, he logged onto Twitter and began asking how to get into Raqqa, he said. Someone replied with a number and he dialed it.

"They asked me what hotel I was in and what room number I was in," he said. "They said they'd come in a bit and I was picked up."

A Syrian cab driver drove him and another recruit, a Tunisian, to Turkish smugglers, who took them and three others to a "safe house" in Harran, he said. The next stop was a border crossing where the five were told to run like hell into Syria.

Turkey has a large military and police presence on the border but has been accused of turning a blind eye, until recently, to jihadi recruits who have flooded into Syria.

In Mo's case, he said, Turkish border patrol guards caught three of the five in his group and "beat us really bad…kicking, a metal rod, the butts of their [AK-47s]," Mo said. "They broke my phone, they were burning our documents."

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Then, Mo claims, the guards inexplicably let the three go, telling them at gunpoint to head for Syria. Mo, who said he had lost his glasses and was wearing a medical boot on his foot for a bone spur, "just ran."

"I remember zigzagging into the darkness," he said. "I didn't even know what was ahead of me."

He said he arrived in a Syrian village torn up from barbed wire. He and others who crossed the border ended up at a house, where the owner gave them water and summoned ISIS members by phone.

Three men — two of them armed — arrived in a white minivan and took them to a schoolhouse in Saluq where a doctor stitched up Mo, he said. It was in Saluq that Mo officially registered as an ISIS recruit, answering questions that were recorded on a computer form in Arabic, he said.

The form was one of more than 4,000 ISIS personnel files that NBC News obtained from another defector — and one of 15 men that we identified as American citizens or residents.

The form included his address in Brooklyn, phone numbers for his father and brother, a list of credit cards and ID he left as a deposit, and the name of his Twitter contact.

It also noted that he attended Columbia, which it noted "is for the clever." And chillingly, it said that Mo had an idea for ISIS — "a project to break down [i.e. crash] aircraft, which could be examined."

Mo's explanation for the notation is that he was so desperate to avoid the ISIS battlefield, he decided to portray himself as someone with technical knowledge so he wouldn't be categorized as a fighter.

Former FBI agent Ali Soufan, who now runs the security firm The Soufan Group, is skeptical of that account.

"This is baloney," Soufan said. "This is a person who went to join ISIS with terrorist intentions on his mind."

But Mo insists that he had no violent intentions and was, in fact, appalled by the "bloodthirst" he saw around him as he was packed off to a camp on sharia law and military training in towns south of Raqqa. He recalled one man proudly calling himself a "terrorist" and another talking about getting a slave girl.

"It was just not the Islam I grew up with," he said.

ISIS militants line up in a still from a propaganda video. Militant video via Abaca

The recruits were taught how to use an AK-47, though only given three bullets for target practice because ammunition was running low, he said. An instructor demonstrated how a bomb works on a whiteboard.

A man who had volunteered to be a suicide bomber gave a lecture and showed off his suicide belt. "People were gravitating toward it, touching it like it's an exhibit, while I was backing away slowly," Mo said. "People were just in awe of it."

On the streets, he saw random beatings and arrests of civilians. Passing through Raqqa, he came upon a terrifying sight, "severed heads placed on spiked poles."

"You could see madness in their eyes," he said of his fellow ISIS members. "People who just had a readiness for violence."

Mo said he ended up in the town of Munbij, working as a guard at a gate and then in an accounting post under the very watchful eye of a half-dozen emirs.

Desperate to escape, he informed on a thief to win favor with his superiors and was rewarded with more trust and freedom, including access to Internet cafes where he was able to download maps, he said.

He heard about a gate that was not under ISIS control, and when he was left alone for an hour one Friday at the end of October, he decided to make his move.

"I packed whatever I could, whatever I thought I'd need and just literally walked out," he said.

He got a lift on the back a civilian motorcycle, then took a taxi to the bus station and caught a bus to Raqqa.

"I'm not supposed to talk about the specifics of how I escaped," he said. "However, what I can say is I met someone who helped me."

During a 12-hour wait in an Internet café for a stranger who could supposedly get him into Turkey, Mo said he tried to email his parents – who had received just two calls from him in nearly five months — an "I-might-die-tonight letter."

"That night I realized what terror was," he said.

When he got to Turkey, he said, he went straight to the U.S. consulate and turned himself in.

Back in the U.S., the FBI had some idea that the young man they had visited in the spring might try to come back because he had emailed the agents from Syria. The bureau worked with federal prosecutors in the Eastern District of New York to obtain a warrant for his arrest.

After being escorted back to the U.S. and arrested by the FBI, Mo signed a cooperation agreement, pleading guilty to providing material support to a terrorist organization and receiving military training from a terrorist organization, FBI officials said.

The two charges carry between 10 and 25 years in prison, but Mo will not be sentenced until "his cooperation is complete," the officials said. The court may consider "any substantial assistance" he provided in deciding whether to give him a lighter sentence, they said.

"That night I realized what terror was."

FBI officials said they could not talk in detail about the work Mo has done because "they relate to ongoing investigations." However, they did say that he "has provided reliable information about the identities and activities of other ISIS members."

Soufan, the security expert, said defectors like Mo are valuable assets to investigators.

"We need to figure out who are these returnees and why did they go in the first place? Why did they come back? And did they come back because they were really surprised with what they saw over there? Shocked? Disgusted with it? Or did they come back because they wanted to be sleeper cells," he said.

American defectors can also help create a "counter-narrative" campaign to dull the slick marketing that ISIS directs at foreign Muslims and converts, Soufan added.

"People inside the bubble of ISIS probably won't listen," he said. "But people who are contemplating going inside the bubble… they might listen."

Mo claims that's now his goal.

"I want to be the voice that helps deter extremism and really attack false ideology at its core," he said.

"It's something I'm absolutely committed to and my parents know my commitment and the government knows my commitment and I hope America can see my commitment as well."