LOS ANGELES — Police officers in the Fontana Unified School District in San Bernardino County were armed recently with semiautomatic rifles, drawing sharp criticism and sparking an effort to ban such weapons on school campuses.

The Colt rifles, which cost about $1,000 each, are kept in safes when officers are on campus and will be used only in “extreme emergency cases” like the massacre in Newtown, Conn., Superintendent Cali Olsen-Binks said.

The district purchased the rifles in October and received them in December, before the tragedy in Newtown, where a gunman killed 26 people — 20 of them children — at an elementary school. The shooting sparked debate on whether armed-school guards could prevent these types of tragedies.

The purchase was not spurred by a specific event, Fontana Unified School District Police Chief Billy Green said. The rifles are designed to increase shooting accuracy and provide the 14 officers with more effective power against assailants wearing body armor, Green said, adding that those capabilities are necessary for officers to stop a well-armed gunman.

“If you know of a better way to stop someone on campus that’s killing children or staff members with a rifle, I’d like to hear it,” he said. “I don’t think it’s best to send my people in to stop them with just handguns.

Fontana school police bought the guns for about $14,000, which fell below the threshold that requires school board approval. Board member Leticia Garcia said the police chief and superintendent should have alerted the five-member board and held a public hearing on the issue.

Garcia, whose son attends Fontana High School, said she is working with local state legislators to draft a bill that would keep school police departments from taking these types of weapons onto campuses.

Garcia worries that bringing such a weapon on campus could lead to it falling into the wrong hands. An officer could be overtaken or someone could gain access to the safe, she said.