Police searched Crosby Elementary in Harvard Monday morning after a custodian found a used shell casing from a bullet.
Administrators were already planning a walk-through of the Harvard Community School District 50's buildings after the recent school shootings in Connecticut.
The discovery of the shell casing in the school's cafeteria prompted school leaders to do a more comprehensive inspection with the Harvard Police Department, said Superintendent Lauri Tobias in a statement.
Students stayed in their classrooms and continued on with their day.
Nothing was found and everyone was safe, Tobias said.