Police said they first became aware of the threats made toward North Elementary School in Des Plaines in September.
There was an anonymous tip about a post on TikTok. The poster said the Uvalde, Texas school shooter was their "idol," and they threatened to "shoot up" North Elementary.
Investigators submitted a search warrant to TikTok, and that led them to 18-year-old Esmeralda Vargas, a former student at the school.
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Vargas was arrested this week and charged with felony disorderly conduct, police said.
No further information was immediately available.