特殊能力があるかどうかは分かりませんが、普通の人間以上の力はあった事でしょう。

、。

I don't know whether they had special abilities, but they must have had strength beyond that of an ordinary person.

Literal

Special-ability [subject-が] exists-whether [topic-は] don't-know but, ordinary person-beyond strength [topic-は] existed thing [conjecture-でしょう].

かどうか embeds a yes/no question as a noun clause. 以上の uses 以上 (more than/beyond) as a pre-nominal modifier — 普通の人間以上の力 = 'strength beyond that of an ordinary person.' The ending ~た事でしょう nominalizes the past-tense clause and でしょう expresses conjecture about what was probably the case.