Proviso West High School was placed on lockdown Wednesday afternoon in west suburban Hillside after a bullet was found on the floor in the cafeteria.
The school, located in the 4700 block of West Harrison, was on a "precautionary lockdown" as authorities conducted a search of the building. A juvenile was arrested for possession of firearm ammunition, police said.
"We do not believe that any student or staff member is in imminent danger," the school said in a statement during the lockdown. "However, these measures are being taken to ensure the safety and security of our students and staff. We will send additional messages once the situation has been resolved."
Students were locked into classrooms and locker rooms, and parents were alerted by phone. School dismissal was expected to be delayed.
Several police cars and an emergency vehicle were seen outside the school.
Helicopter footage of the scene showed officers and police dogs sifting through bags of trash from school dumpsters.
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