The message was discovered Tuesday in Schroeder Hall, one of the main classroom buildings, The (Bloomington, Ill.) Pantagraph reported.
University officials said the threat was probably a hoax but security measures would remain in place at least for a couple of days.
Jay Groves, the university chief of staff, said nothing suspicious was found in several searches of Schroeder Hall.
He said the threat was not considered serious enough to justify an immediate text alert to students, faculty and staff.
"We do not want to overreact to this situation and add credibility to the individuals and copycats who commit such acts," ISU President Timothy Flanagan said in an email to the university community. "Still your safety is our first priority, so I felt it necessary to take these extra safety precautions, alert you quickly and keep you updated."