A student at a Chula Vista middle school was arrested Thursday, accused of bringing an unserialized “ghost gun” onto campus, Chula Vista police said.

Officers were called in after a Rancho del Ray Middle School staffer found an unspent bullet on the campus about 10:50 a.m., police said in a news release.

Police and staffers spoke to several students and learned a student had been seen discarding an object near the school’s fence line, officials said. With that information, the school went into secure campus mode while officers searched the area.

The search turned up a gun hidden under the bushes along the fence line and an additional bullet. The gun — which did not have a serial number and was therefore untraceable, a kind of weapon known as a ghost gun — was not loaded, police said.

A student was arrested. Police did not provide further information about the person, who is a juvenile.

“The Police Department commends the students and staff who promptly reported concerning information, allowing officers to respond quickly and safely,” police said.