It is best to bring this up via a flag 'other.' Flag one of the affected posts and describe the situation briefly; to flag one post suffices, we can see which other posts were downvoted then. (I see this was just said in a comment, too; so I confirm this.)
However, moderators do not know the origin of individual votes either. But, we can access reports about voting that do allow to identify such behavior sometimes. If the voting seems abusive, we do take action.
In passing, this is also addressed in the general post on this subject: When are serial downvotes classified as anomalous? What to do about a sequence of downvotes?