SANTA ROSA, Calif. (KRON) — Three Santa Rosa High School students were arrested on Thursday after they allegedly brought a firework to school and set it off. The Santa Rosa Police Department said all three students were 15-year-old boys.

Police were called to the school at 10:35 a.m. for reports of people believing they heard an explosion. Upon arrival, officers determined that there was no active threat to the school, per SRPD.

SRPD said the firework was detonated in an outdoor passage between two buildings. Students and staff originally thought it might be a bomb or an active shooter.

Students fled the area, and school officials ordered a shelter-in-place while officers searched the area.

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With help from school officials, officers found the two students that detonated the firework and a third who brought it to school. All three were arrested on a misdemeanor count of possession of an explosive device.