Agents with the Department of Homeland Security searched two university residences with a warrant Thursday evening. No one was arrested and it was unclear whom the authorities were searching for.

A top U.S. Justice Department official says the department is investigating whether Columbia University concealed “illegal aliens” on its campus.

Between classes, when throngs of students stream into and out of Columbia University ’s campus, the security lines can back up like a TSA checkpoint. There’s a switch-back path of metal fencing that leads into a white tent, where two sets of guards check and then swipe campus ID cards before letting students in and out.

The Department of Homeland Security said a Columbia University student who had her student visa revoked self-deported. Another Palestinian student was arrested for overstaying her expired visa.

Columbia University says it has expelled or suspended some students who took over a campus building during pro-Palestinian protests last spring and temporarily revoked the diplomas of some students who have since graduated.