Somehow I didn't imagine them using the term "suicide" mission. ?
They refer to them in the letter as those who desire to volunteer for "Al AA'mal Al Feeda'ee Al Intihari" to liberate Palestin and to strike the America interests
AL AA'mal: The Act or the Mission.
Al Intihari= Suicidal
Al Feeda'ee = Some who sacrifices himself, i.e. Suicide. I am sure you heard about Feedeyeen which is derived from Feeda'ee
الأعمال الإنتحاري الفدائي
as phonetically written by jveritas as "Al AA'mal Al Feeda'ee Al Intihari" does mean "Suicidal Fighter Operations".