Answer:
The investigators were making use of the guilty knowledge test.
Explanation:
The guilty knowledge test, as the name suggests, uses the knowledge that only a perpetrator would know to test if that subject could be guilty of that crime. A polygraph examiner watches the subject's physiological arousal under the belief that certain details will provoke reactions. In this particular case, the real criminal would know that the crime was committed with a knife, not a letter opener. Therefore, if the subject being examined is the perpetrator, he will not react to "letter opener" as he will to "knife".