What would you do in this "fabricating evidence" dilemma?

You're a detective. You are 95% sure someone is the serial killer that's been going around killing people and their current count is 6 people. They are really good at hiding their tracks and you've already brought them in for questioning many times. Their tone, body language, and their history all point to them being the killer, but you don't have the evidence to put them away. Would you fabricate evidence with the 95% chance in your gut that it's them? Or is it innocent until proven guilty and let the potential killer roam free?
What would you do in this fabricating evidence dilemma?
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You have to assume you can't catch him because he's the type to wait patiently until he's sure surveillance is over and he's in the green. You only have the option of planting evidence or letting him walk due to the time spent on his case and his lawyer forcing you to sign a immunity agreement.
What would you do in this "fabricating evidence" dilemma?
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