Nigerian Islamist group Boko Haram released a group of thirteen prisoners, police and lecturers, kidnapped last year, announced the Nigerian presidency on Saturday without giving details of the operation.

A group of ten policewomen were kidnapped last June during an ambush in the vicinity of Maiduguri, the capital of Borno State in northeastern Nigeria.

Police initially denied the kidnapping, but the Boko Haram group released a video several weeks later showing women begging to be released.

Three lecturers from Maiduguri University were kidnapped last July during an oil exploration mission in the north-east of the country as well.

At least 69 people were killed during Boko Haram’s ambush on the team of explorers during one of the bloodiest attacks last year.

“Their release follows a series of negotiations led by President Buhari and supported by the International Committee of the Red Cross (ICRC),” Presidential spokesman Garba Shehu said in a statement.

The 13 people are en route to Abuja with the assistance of the Nigerian army and air force, the Nigerian authorities said.

President Muhammadu Buhari claimed in December 2015 that Boko Haram was “technically beaten” in the face of the Nigerian army’s recovery.┬áIn late November, he said that village attacks and suicide bombings, which have once again accelerated, are only the “last kicks of a dying horse”.

The insurgency has left at least 20,000 dead and 2.6 million displaced since 2009.