Police say they found a 15-inch chef's knife in a student's back pack when they responded to a locked down school where someone wrote "I have a gun" on a rest room wall.
Ambridge police Chief James Mann doesn't believe the knife was related to the gun threat at Ambridge Area High School on Wednesday.
Police searched every student and their back packs after the gun graffiti prompted officials to lockdown the school about 25 miles northwest of Pittsburgh.
Police say the student told them he put the knife in his back pack after arguing with another student a couple of months ago, then forgot it was there.
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That 15-year-old boy will be prosecuted in Beaver County Juvenile Court.
The chief says no students were in danger during the lockdown.
On April 9, a student at another suburban school, Franklin Regional High School in Murrysville, was charged with attempted homicide and aggravated assault after he allegedly used two kitchen knives from his home to stab 20 students and a security guard.