Police are trying to figure out how a bullet got into Leonia High School.
The discovery on Wednesday led officials to lock the school down for two hours.
Students were sent to the gymnasium while police searched the building after a student notified a teacher about the bullet.
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The lockdown was lifted at 3:40 p.m. and students were sent home.
Police Chief Thomas Rowe tells The Record newspaper police don't believe the student who reported finding the bullet was responsible for bringing it in the building.