Island Lake's Matthews Middle School was locked down for more than four hours Thursday after a bullet was found on the floor, police said.
The 9 mm round was live and still attached to a shell casing, police Sgt. Nick Deuter said. Marks indicate it may have been a dud round that didn't fire earlier.
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The school, at 3500 Darrell Road, was locked down starting about 8 a.m. as a safety precaution, authorities said.
In an email to parents, Wauconda Unit District 118 Superintendent Daniel Coles said a student found the cartridge on a hallway floor at the start of the school day.
The student immediately gave it to a teacher, who reported the situation to administrators, Cole wrote. They called police.
The ensuing investigation included a canine search, searches of lockers and backpacks and a comprehensive review of security camera footage. No other ammunition was found, nor were any weapons found, Deuter said.
"We did a thorough search of the school," Deuter told the Daily Herald.
Students and teachers were allowed to resume normal daily duties about 12:30 p.m., Coles wrote.
No arrests were made.
The situation remains under investigation.
If you have information for police, call (847) 526-2100.
A Matthews Middle student was arrested in March 2012 after bringing a loaded handgun to school. No one was injured in that incident.
The student's father was arrested and charged months later, too.