To use the guilty-knowledge technique, the polygrapher must know a fact about the crime that could only be known by the:

guilty party

Explanation:

The Guilty Knowledge Test is a questioning technique used when doing a  polygraph examination which tries to see if the client has “guilty knowledge” by measuring their responses while responding to a series of multiple choice questions. According to my research on the GKT, I can say that based on the information provided within the question this would only work if the polygraph-er knows facts that could only be known by the guilty party.

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