Kaneland High School was placed on alert Wednesday afternoon after an employee reported finding a spent 9 mm handgun shell casing in a hallway near the cafeteria at the Maple Park school.
Students and staff were told to stay in their classrooms at 12:15 p.m., but were allowed to continue activities in those classrooms. The district calls this a "hold in place."
Students who were in the cafeteria were taken to the auditorium. They had to leave their belongings in the cafeteria.
Sheriff's deputies, including the school resource officer and a police dog, searched the building and did not find any other casings or other threats, according to Kane County Sheriff's Lt. Pat Gengler.
The hold in place order was lifted at 1:45 p.m.
Authorities continue to investigate where the shell came from.