When protests over the Israel-Hamas war took root on Columbia University's campus last spring, Mahmoud Khalil became a familiar, outspoken figure in a student movement that soon spread to other U.S. colleges.

Mahmoud Khalil, a 29-year-old legal resident of the United States, was arrested and detained by federal agents Sunday, after his green card was revoked by the Trump administration.

Khalil’s arrest could have a chilling effect on on- and off-campus protests involving people who aren’t citizens, said Columbia University immigration law professor Elora Mukherjee. Already, she said, it had provoked panic and fear among immigrant students on campus.