PHOENIX – A teenager was arrested Friday afternoon after he brought an AR-15 rifle to school with extra ammunition in his backpack and lunch box, police said.

Phoenix police officers said they responded to Bostrom High School in Maryvale just before 1 p.m., where they found the student with the AR-15 semi-automatic rifle and ammunition. Officers arrested the teenage boy, who wasn’t identified by authorities, at the school’s main office under multiple “serious felony charges,” police said.

The principal said in statement that administration learned about a possible weapon on campus at lunch and placed the school on lockdown. The campus will have additional safety measures in place on Monday “out of an abundance of caution,” Principal Michelle Gutierrez de Jimenez said.

Bostrom is an alternative high school with a small learning environment, according to the Phoenix Union High School District website.

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Police said they were working with school and district officials to investigate the incident.

“We commend those who originally reported the possibility of a weapon on school grounds to adults on campus who immediately called police,” police said in a statement released Friday.

At least nine states have banned sales of an array of semi-automatic weapons, including AR-15s, that have become known for their use in some of the deadliest mass shootings in the country. The rifles are semi-automatic because they fire one bullet per pull of the trigger and automatically reload for another shot. A fully automatic weapon would continuously fire as long as the trigger is pulled until the supply of ammunition runs out.