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CALUMET TOWNSHIP — Sheriff's detectives arrested a 13-year-old boy at his home shortly after 5 p.m. Friday — just hours after he allegedly posted to social media plans to shoot up Lake Ridge Middle School on Monday. 

"Because of this day and age, and what’s going on in the nation, we have to take these threats seriously," Lake County Sheriff Oscar Martinez Jr. said Friday night. 

The boy was expelled three weeks ago from Lake Ridge middle school for disruptive behavior, he said. 

At about noon Friday, the teen posted on a social media site that he planned to shoot up "whoever showed up" to school on Monday, Martinez said. 

Martinez said friends and acquaintances of the boy brought the threatening post to a school resource officer's attention and the Lake County Sheriff's Department assigned its Special Victims Unit to investigate, Martinez said. 

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The 13-year-old, whose name is not being released because of his juvenile status, was booked at the Lake County Juvenile Center on felony charges of intimidation using a firearm, a level 6 felony, and harassment by making threats through a website, a level 5 felony. 

"These are serious threats regardless of if they are done out of spite or anger. We take them all serious," Martinez said. "It's time for parents to sit down with their children, and discuss what they do on social media (have consequences)."

Recent arrests booked into Lake County Jail

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