、。
However, the survivors didn't know that fact.
Literal
However, survivors-[plural-達] [topic-は], that fact [object-を] did-not-know.
Narrative past register. 生存者達 'the survivors' uses the plural suffix ~達 — optional in Japanese (singular/plural is usually not marked) but added here to emphasize the group. 知らなかった 'did not know' is the plain past negative of 知る. The sentence has a classic narrative-cliffhanger rhythm: introducing a contradiction between appearance and reality.