(CNN) -

Thirty-five Nigerian policemen are missing following an insurgent attack Saturday on a police training school in one of the most volatile parts of the African nation, police said.

A search-and-rescue effort has been launched after the third attack this month the Gwoza Training camp in Borno State, according to a statement posted online by the Nigeria Police Force.

It was not immediately clear whether anyone had claimed responsibility.

Afterward, the force's Inspector-General Suleiman Abba ordered that security be tightened around all police facilities in the country.

It's not known what happened to the missing officers, though the police force statement did say there is a "very promising prospect of locating them."

"While enjoining all citizens to remain calm and join hands with the security forces in the collective fight against insurgency, the IGP restates his determination to work closely with other security agencies to stem the tide of terrorism in the country," added the police in their statement out of Abuja, Nigeria's capital.

Nigeria has been beset by violence for some time, particularly in the northeastern states of Borno, Yobe and Adamawa.

Much of it has blamed on the Islamist militant group Boko Haram -- whose name means "Western education is sin" in the local Hausa language. The group has bombed schools, churches and mosques; kidnapped women and children; and assassinated politicians and religious leaders alike.