Crowds of masked student protesters raging against the war in Gaza filled the Columbia University lawns last spring, while counterprotesters and journalists surrounded the tent city that had been erected there.

One man stood out.

He was Mahmoud Khalil, a graduate student in his 20s, older than most of the students around him. Mr. Khalil, a Syrian immigrant of Palestinian descent, quickly emerged as a vocal and measured leader during rallies and sit-ins, doing on-camera interviews with the media in a zip-up sweater.

And he was unmasked. Many other international students wore masks and kept to the background of the protests, for fear of being singled out and losing their visas.

His wife worried. “We’ve talked about the mask thing,” Noor Abdalla, a 28-year-old dentist from the Midwest, said in an interview last week. “He always tells me, ‘What I am doing wrong that I need to be covering my face for?’”

Mr. Khalil was a negotiator on behalf of Columbia University Apartheid Divest, the main coalition of protesting student groups, and one with its own spectrum of attitudes toward violence and dark rhetoric.

His decision to quite literally be the face of a deeply divisive movement would have huge consequences for Mr. Khalil. He was called out by critics by name on social media, and on March 8, seven weeks after the inauguration of Donald Trump, federal agents arrived at his door. He was swiftly taken to a detention center in Louisiana, where he is still being held for what officials have described, without providing details, as leading activities aligned with Hamas, an allegation he has denied.

Mr. Khalil’s friends and family have expressed outrage at his detention and possible deportation. But they also say they are not surprised by his activism in a movement that he was born into, nor his relatively calm presence amid a swarm of noise.

As he moved through the world, Mr. Khalil could often come across as the adult in the room. And to one who had known him as an office mate in an earlier time, his role in front of microphones and wielding a bullhorn came unexpected.

“He’s very sort of mild mannered,” said Andrew Waller, a former colleague who worked with Mr. Khalil in Beirut at the British diplomatic office for Syria. “Seeing him in more of a sort of leadership or spokesperson role, I guess was a surprise.”

Mr. Khalil arrived at Columbia University at the end of a long and winding journey. His Palestinian origin story was written and ended before he was born.

His grandparents were from a village near Tiberias, a city on the western shore of the Sea of Galilee in Palestine before it became part of the state of Israel. They were forced to flee in 1948 during the wars preceding Israel’s establishment, Mr. Khalil has said, settling with other members of their large family in southern Damascus in Syria, in a Palestinian refugee enclave. It was there that Mr. Khalil was born in 1995.

In the early 2010s, he fled the Syrian conflict to Lebanon, where he arrived alone and broke. He worked in construction to make enough money to pursue an education, according to his friend Ahmad Berro, who met Mr. Khalil while the two were studying at Lebanese American University. Mr. Khalil graduated in 2018 with a degree in computer science.

While in Lebanon, Mr. Khalil worked with Jusoor, a Syrian American educational nonprofit. There, in 2016, he met the woman who would become his wife, a U.S. citizen of Syrian descent.

In 2018, he began working on programs related to Syria for the British diplomatic office in Beirut. He eventually oversaw a scholarship program for foreign students to study in Britain. His work was informed by his personal experiences of fleeing Syria and his opposition to the government there, Mr. Waller, his former colleague, said.

After about four years, Mr. Khalil set his sights on the United States and applied to a few graduate schools. He hoped to be accepted at one in particular, Columbia University and its School of International and Public Affairs.

He was accepted and enrolled in January 2023.

He saw it as a huge win, not only for himself, but for his fellow refugees, said Lauren Bohn, a journalist who met Mr. Khalil in Beirut and spent time with him after his admission to Columbia. “He said, ‘This will really help me serve all the others who aren’t going to be able to get this chance.’”

He had been at the university for some nine months when everything changed on Oct. 7, 2023.

A campus in turmoil

Students at Columbia turned out for protests immediately after Hamas’s attacks on Israel. Some were quiet calls for peace, others more raucous. Pro-Palestinian and anti-Israel chants rang through the campus, rattling many Jewish students.

Mr. Khalil was on the front lines with Palestinian activists, bracing for a counterattack from Israel that was imminent. In a video from Oct. 12, five days after the attacks, he is seen atop another person’s shoulders, shouting “Free Palestine!” into a bullhorn.

Months of protests followed. Then, in April 2024, pro-Palestinian students established an encampment at the center of campus. They demanded that the university divest from what they called “all economic and academic stakes in Israel,” including Columbia’s dual-degree partnership with Tel Aviv University.

The rows of tents pitched on Columbia’s iconic, grassy lawns inspired similar protests at universities across the United States. They became a flashpoint after Columbia’s president called the New York City police to campus, leading to the arrests of more than 100 people. As the protests intensified, some Jewish students complained about feeling unsafe. Some heard anti-Zionist chants as threatening to them personally. Those accounts reached Congress, where Republicans derided the protests as antisemitic and Columbia as out of control.

When negotiations began between the protesters and the university, Mr. Khalil emerged as a lead spokesman for the students. The two sides met day and night. A Columbia administrator who negotiated with him described Mr. Khalil as thoughtful, passionate and principled, sometimes to the point of rigidity. He got his back up when he felt he wasn’t being taken seriously. Mr. Khalil was also a face of the protesters for the news media, where he was sharply critical of the university, stepping confidently up to banks of microphones where reporters from CNN, Spectrum News NY1, The Associated Press and The New York Times and elsewhere recorded him confronting the school that had brought him to New York.

“It’s very clear the university does not want to criticize Israel in any way,” Mr. Khalil told a gaggle of journalists gathered near the encampment last spring.

On another occasion, at a discussion sponsored by the coalition of student protesters, he remarked that whether Palestinian resistance was peaceful or armed, “Israel and their propaganda always find something to attack.” He added, “They — we — have tried armed resistance, which is, again, legitimate under international law.” But Israel calls it terrorism, he said.

Those comments were highlighted as justifying terrorism by pro-Israel activists on a webpage about Mr. Khalil that had been compiled by Canary Mission, a group that says it fights hatred of Jews on college campuses and that pro-Palestinian protesters say has doxxed them.

Still, Mr. Khalil repeatedly told friends, as he had his wife, that he saw no reason to wear a mask. What were they going to do to me? he asked.

Once, when the number of tents rose to more than 100, including on a second lawn near the School of Journalism, administrators turned to Mr. Khalil. They made him an offer: Remove about 20 tents, they said, and we’ll ensure that the university’s trustees continue to discuss your demands.

Mr. Khalil countered, agreeing to remove a few less than the administrators wanted, according to one administrator present at those talks, who spoke on condition of anonymity to discuss private university negotiations.

Within minutes, 17 tents vanished and the second lawn was emptied. This response burnished Mr. Khalil’s reputation as a good-faith, if demanding, negotiator.

Other times, he stood fast. Late in the protests, when the university offered concessions and the threat of the police arriving to clear out demonstrators was looming, Mr. Khalil pushed back. We don’t want your concessions. The police? Let them come.

Then they did.

New protests, new president

After a faction of protesters took over Hamilton Hall, a campus building, on April 30, barricading doors and trapping custodians inside, scores of police officers descended on the university. They arrested dozens of pro-Palestinian demonstrators and cleared the hall.

Mr. Khalil was not accused of being in the hall. He had been suspended by the university just before the building takeover, accused of refusing to leave the encampment, along with many other pro-Palestinian activists, and then was quickly reinstated. But there were no more negotiations, and the protests ended for a time.

Columbia slowly ceased being the global flashpoint for campus unrest. Mr. Khalil focused on finishing his courses and looking for work after graduation.

He and Ms. Abdalla married, and he obtained a green card, giving him permanent residency in the United States.

Last summer, the couple learned that they were having a baby. Mr. Khalil was excited, his friends said, getting their apartment ready even as the couple looked ahead toward moving after he earned his degree.

“He did everything, basically,” Ms. Abdalla, now eight months pregnant, said. “He did all the cooking, he did all the cleaning. He did the laundry. He wouldn’t let me touch anything.”

He finished his coursework for his master’s degree from the School of International and Public Affairs in December. But he remained aware of protests still bubbling up at Columbia and at Barnard College, across Broadway.

In January, protesters stormed into a Columbia classroom, and two Barnard students were later expelled that month for their roles that day. It was a flashback to the turmoil of the previous spring. While Mr. Khalil was not present, he was soon drawn back in.

Days later, President Trump, newly inaugurated, issued an executive order promising to combat antisemitism and prosecute or “remove” perpetrators of such views.

The same night, an X account of a Zionist group singled out Mr. Khalil. It accused him, without evidence, of saying that “Zionists don’t deserve to live,” and said that the federal Immigration and Customs Enforcement agency had his home address. “He’s on our deport list,” the post said.

It included a video of Mr. Khalil speaking in a CNN interview, during which he made no such statement. Mr. Khalil has said he had “unequivocally” never spoken those words — another student had, and was expelled.

Mr. Khalil saw himself and other student protesters as victims of doxxing, finding their personal information spread on social media. On Jan. 31, he emailed Columbia administrators asking for protection for international students, such as himself, who he said were facing “severe and pervasive doxxing, discriminatory harassment and very possibly deportation.” A Columbia spokeswoman declined to comment on communications from Mr. Khalil.

Jasmine Sarryeh, a close friend, tried to allay his concerns and told him he would never be deported. Now she feels like she let him down.

“I didn’t think to expect that this would happen,” she said in a recent interview.

‘Suspected Foreign National’

On March 5, in response to the expulsion of the Barnard students in January, protesters dressed in kaffiyehs and wearing masks descended upon the college’s library. It was a Wednesday, and Mr. Khalil turned from his baby preparations and attended as well, maskless again.

It was the beginning of a four-day stretch that would end with Mr. Khalil in federal detention.

Videos on social media depict him at the library holding a megaphone — and, at one point, using it to amplify the Barnard president, who is speaking over a cellphone. When the protesters are asked if they want to speak with the president, Laura Rosenbury, Mr. Khalil gives them an encouraging thumbs up. They respond in unison: “Yes!”

Critics of the protests immediately began posting videos and images of Mr. Khalil on X, calling him out by name.

One post included an image of his face circled in red with the label “Suspected Foreign National.”

Then, Shai Davidai, an Israeli Jew and Columbia professor banned from campus in October after he was accused of harassing employees, reposted that image and tagged another X account. It belonged to Marco Rubio, the secretary of state, who had just posted a threat to deport Hamas supporters.

“Illegally taking over a college in which you are not even enrolled and distributing terrorist propaganda should be a deportable offense, no?” Mr. Davidai wrote.

Separately, Shirion Collective, a group that says it exposes antisemitism, has said that it sent the Department of Homeland Security a legal memorandum advising the “detention and removal” of Mr. Khalil.

Mr. Khalil saw some of the posts online and panicked. He was being singled out for deportation directly to the very official with the power to set that process in motion.

On Friday, March 7, he again wrote to Columbia administrators and described a “vicious, coordinated and dehumanizing doxxing campaign” against him.

“I haven’t been able to sleep, fearing that ICE or a dangerous individual might come to my home,” he wrote.

That fear would be realized the next day.

‘Let’s bring him in.’

Mr. Khalil and his wife were out with friends on Saturday night, March 8. When they returned to their Columbia apartment, a man in plain clothes pushed into the lobby behind them. Ms. Abdalla felt her husband tense.

“He knew something was wrong,” she said.

I’m with the police, the man said. You have to come with us. More officers arrived in the lobby. Ms. Abdalla hurried up to their apartment to get her husband’s green card. She reminded the officers that he was a permanent citizen.

“‘This guy has a green card,’” she heard the officer say on his phone. “And then the guy on the phone with him told him, ‘Let’s bring him in anyway.’”

In a video recording of the arrest, she is heard asking the officers repeatedly to identify themselves and to specify what charges her husband was facing. She rushes after the officers into the street as they ignore her questions.

It remains unclear what exactly Mr. Khalil is believed to have done. He is accused by the White House and others of organizing protests, such as the one in the Barnard library, where participants distributed fliers promoting Hamas. A flier that was shown in online postings from the library said it had been produced by the “Hamas Media Office.” It was titled “Our Narrative” and listed Hamas’s code name for the Oct. 7 attacks, with an image of fighters standing on a tank. It is unclear whether Mr. Khalil knew the fliers were there.

“I can wholeheartedly say that I know that he did not touch those fliers,” said Mr. Khalil’s friend, Maryam Alwan. “But just because he had his face out, people are trying to pin everything on him.”

His lawyers also denied that he had distributed the fliers at Barnard.

Mr. Waller, his former colleague in Lebanon, said the depictions of Mr. Khalil that he had seen in the news media did not line up with the friend he knew.

“The idea that he’s somehow a political extremist or a sympathizer with terrorist groups or whatever just sounds totally outlandish,” he said. “If you know him and you know his character, it just feels like a sort of obvious smear.”

There are circumstances in which permanent residency status in the United States can be revoked — if, for example, the resident is convicted of a crime. But Mr. Khalil has not been accused of any crime. Instead, Secretary Rubio has cited a little-used statute as the rationale for Mr. Khalil’s detention. The law says that the government can initiate deportation proceedings against anyone whose presence in the country is deemed adversarial to the United States’ foreign policy interests.

Mr. Davidai, the professor who tweeted the photo at Secretary Rubio, said in an interview that he believed Mr. Khalil was entitled to due process under the law. But, he added, it does not so much matter whether Mr. Khalil personally handled fliers promoting terrorists, if the group he represented did.

“When you lead an organization, you are accountable for your organization’s actions,” Mr. Davidai said. “When you lead an organization that openly and proudly supports a U.S. designated terrorist organization, you are accountable to the spreading of propaganda.”

Mr. Khalil has said he was never the planner and leader of the pro-Palestinian protests; he has consistently described himself as a spokesman and negotiator for a coalition of student groups.

Resolving this was not the job of the agents who came to his lobby that Saturday night. They handcuffed Mr. Khalil, led him to a car waiting outside and drove him away.

Katherine Rosman, Sharon Otterman, Jonah E. Bromwich and Michael LaForgia contributed reporting. Kirsten Noyes contributed research.