NAIROBI, Kenya (AP) — Eighty U.S. Air Force personnel have arrived in Chad and have begun their mission to help locate nearly 300 schoolgirls kidnapped in neighboring Nigeria, a U.S. military spokesman said Thursday.

The girls and young women were kidnapped on April 15 from a school in the northeast Nigerian town of Chibok by an Islamic extremist group known as Boko Haram. The group’s leader has threatened to sell most of the estimated 276 schoolgirls still being held into slavery unless the Nigerian government releases detained militants. Reports say some girls were taken across borders into Chad and Cameroon.