LAGOS, Nigeria (AP) — Nigerian police have arrested three “key suspects” over the abduction for ransom of 121 students from a school in the northern Kaduna state, authorities said Thursday.

Arrests of suspected school abduction masterminds in the country's northwest and central regions are rare.

Assailants had stormed the Bethel Baptist High School on July 5, seizing the students from their hostels. All but 21 of them have been released, including 10 freed last week.

Police spokesperson Frank Mba said Thursday that one assault rifle was recovered from each of the suspects. Security experts, parents and rescued students have told The Associated Press that kidnappers often use ransom money to buy weapons.

Africa’s most populous country has witnessed at least ten mass school abductions in the last year during which more than 1,400 students were taken in total, often in remote areas where there is little security presence.

The Rev. John Hayab, an official at the Kaduna school, told The Associated Press that since the school was attacked in July, attacks have continued in various parts of the state by gunmen using hideouts in abandoned forest reserves.

“They feel comfortable and are still coming back to kidnap more people,” he said, adding that parents of affected students are frustrated that “the state is not taking the war to (the gunmen).”