He knows Wilden and Garrett are linked. Holbrook is no dummy. Garrett's photo is huge on the board, and Wilden is linked to it in black. Next to Wilden's photo is written, "Was he looking to solve or conceal? Wilden activity continues after Garrett homicide. Complicit? Extortion?" Meanwhile, near Garrett he has written "Theatre mgr. I.D. of Garrett also fits Wilden timeline."
He's also clued into Ali. Clearly, he realizes that Ali's death may play a part in the more recent murders, including Wilden’s death.
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