The U.S. Justice Department is investigating whether Columbia University concealed “illegal aliens” on its campus, one of its top officials said Friday, as the Trump administration intensified its campaign to deport foreigners who participated in pro-Palestinian demonstrations at the school last year.

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

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.