AURORA, Colo. (KDVR) — An elementary school was briefly put on lockdown due to an employee breaking protocol Thursday morning, Sable Elementary School Principal Megan Ortiz said.

The Aurora Police Department and APS security were called to Sable Elementary after a staff member entered the building through a back door which is against security procedure. Staff members that didn’t recognize the employee quickly placed the school on a temporary lockdown as a precaution, Ortiz said.

“Safety is our top priority, and a lockdown is put into effect as a precaution. During a lockdown, it is routine for police officers to check and clear all classrooms. Our school then safely transitioned into a secure perimeter. That secure perimeter was then lifted and school is now back to normal,” Ortiz said in a letter sent to Sable Elementary parents.

The school has since resumed regularly scheduled activities and learning.