The correct answer is were.
Since English grammar has this thing called the sequence of tenses, which means that if the first verb in the sentence is in the past tense, all following verbs will have to be in the past tense as well.
So here, the first verb is knew, which is the past simple tense of the verb to know, which means that the following verb to be also has to be given in the past. The past simple form of the verb to be for plural is WERE.
