Choose the correct phrase to complete the detective's dramatic reveal.

I finally caught the thief! I _____ it was the butler since the very beginning of the investigation.

The correct answer is have known.

Even though the action started in the past and continues to the present (which often requires the Present Perfect Continuous), the verb know is a stative (state) verb. Stative verbs describe a state of being rather than a physical action, so they are rarely used in continuous tenses. Therefore, we must use the Present Perfect Simple (have known).