It happened Tuesday at Lester Elementary in Downers Grove.
An adult saw the swastika in the morning and alerted school officials.
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In a letter to parents, District 58 Superintendent Kevin Russell said it's believed no students saw it, and called it "deeply disturbing."
He went on to say, "The swastika serves as the most notorious symbol of hate and antisemitism and has no place in our community."
There's no evidence the symbol was created by a Lester student.
Russell said it likely occurred after school, and asked for extra police presence in the area.