Flossmoor Police Chief Carl Estelle said law enforcement officials received a call from Homewood-Flossmoor High School just before 1:30 p.m. for a gun and ammunition found on the floor in the school.
Police said a semi-automatic handgun was found.
The school was placed on a hard lockdown, school officials said.
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A student was arrested, and there are currently no active threats, officials said.
Video taken inside the school shows a black handgun surrounded by bullets on the floor.
Police were on the scene during a staggered dismissal that started at 3 p.m.
Officials did not immediately provide any additional information about the incident.