A sole bullet found in a hallway at St. Charles North High School on Thursday sent students home early and forced the cancellation of all on-campus after school events, school officials said.
School officials said they don't believe there is an imminent threat, but cleared the school and said searched all student bags.
No guns or other weaponds were recovered from the school, at 255 Red Gate Road in the western suburb, said police spokesman Paul McCurtain.
Students were dismissed from school around midday, being told only that there was a "security incident."
"We are merely just exercising caution," School District 303 spokesman Jim Blaney told the Kane County Chronicle.
St. Charles police asked that anyone with information call its investigations unit at 630-377-4435.