YOLA, Nigeria (AP) — Nigeria’s defense minister says it may take years to find all the Chibok schoolgirls kidnapped by the Nigeria-based Islamic extremist group Boko Haram.

The group kidnapped 276 schoolgirls from a school in the northeastern town of Chibok in April 2014. Some escaped and 21 were released last year after negotiations with Boko Haram, but 195 remain missing.

Gen. Manir Dan Ali told Voice of America’s Hausa language service on Tuesday that Nigeria’s military is searching Boko Haram’s hideouts in the Sambisa Forest, a vast area covering parts of three states in the northeast.

He said even the United States needed a long time to find Osama bin Laden, the mastermind of the Sept. 11, 2001, attacks.

Ali spoke after Nigerians last week marked the third anniversary of the schoolgirls’ abduction.