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.

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