Officials said Tuesday that the threatening message was scrawled on a handwritten note inside Schroeder Hall.
Classes aren't being canceled, but administrators say they'll increasing security for the next several days while law enforcement officials investigate the threat. They're limiting access to the building, checking bags and using special-trained dogs that will sniff for explosives.
ISU President President Timothy J. Flanagan posted a message to the school's website, which read in part:
"We must take these communications seriously. After receiving the communication, University police immediately contacted the Federal Bureau of Investigation and other law enforcement agencies. While we have not cancelled classes at Schroeder Hall, we are taking multiple steps to ensure the safety of all students, faculty and staff members who use the building."
ISU Chief of Staff Jay Groves says the school contacted the FBI as a precaution.
Classes at the Normal school began Monday.
(The Associated Press contributed to this report.)