Americans held as prisonersでも問題ありません。
「they were being held prisoner」のholdは「to hold prisoner」という風に考えてもいいかもしれません(一つのphrasal verbとして)。例えばTake hostageや hold hostageも同じような感じです。
They were taken hostage.
They were held hostage.
They were held prisoner.
回答になりますか?
ご丁寧な説明ありがとうございます。いつも感謝しています。
crystal