Illinois State Police say 36-year-old Michelle L. Smith of Jerseyville is charged in Calhoun County with falsely making a terrorist threat.
Authorities accuse Smith of leaving a typed letter in a Calhoun County High School bathroom Monday. Police say the note indicated a bomb had been placed in the Hardin school, which was evacuated. No explosive device was found.
Smith was arrested the next day.
It was not immediately clear if Smith has an attorney. She does not have a listed home telephone number.
Telephone and emailed messages were left with the school district's superintendent.
Hardin is about 40 miles northwest of St. Louis.