The school issued an alert earlier Wednesday for people in the Rubloff Building to stay in offices with the doors locked after reports of a possible gunman inside an 11th floor elevator.
A notice on the university's Web site says police completed a more than two-hour search of the building. It adds that they remain on the scene and continue to investigate.
Northwestern spokeswoman Pat Tremmel declined to provide other details, including whether there were any arrests.
The Rubloff Building houses a number of research and law school offices.