Fēi / jī / shī / zōng / de / yuán / yīn / chéng / le / dāng / dì / rén / xīn / zhōng / de / wèi / jiě / zhī / mí

飞机失踪的原因成了当地人心中的未解之谜。

The reason for the plane's disappearance became an unsolved mystery in the minds of local people.