Even a "suspected" serial killer isn't a serial killer until convicted and jailed as such (innocent until proven guilty) if they had access to the street to try and make their getaway.
The whole "innocent until proven guilty" entitles the accused to certain rights, but it doesn't mean everyone has to act like they are 100% definitely innocent. Taken to the extreme, that would mean you couldn't even arrest someone suspected to being a serial killer until they were convicted. Would make it a little difficult to make them stand trial...
https://en.wikipedia.org/wiki/Prison also says “In American English, the terms prison and jail have separate definitions, though this is not always followed in casual speech.”