Not that I’m fully on board with the theory, but you might be surprised how often “solving” a high profile case is placed above actually getting the right man.

This is a publicity nightmare for the police, and getting someone in custody “achieves” placating the public and key stakeholders.

Repeating things about this kids views on the Uni-bomber and referring to his writings as a manifesto “achieves” diminishing his status as a folk hero.

So while I won’t endorse any particular theory until more evidence comes out, it wouldn’t be the first time putting a scapegoat in jail was deemed more important than letting people think the “perp” got away. Even if the hypothetical real shooter kills again, controlling the narrative can be it’s own goal in cases like this.