Whatever happened to beyond a shadow of doubt? .
I don’t know that such a standard has ever existed. To what are you referring?
First off, convictions are to the “beyond a reasonable doubt” standard, not shadow of a doubt.
Secondly, the narrowed pool of 20 is a pool of persons of interest, for closer investigation. No one will ever be arrested, let alone convicted of a crime for being part of this initial pool.
Once they identify the single person of those 20 who had means, motive and opportunity, police can then obtain a warrant to compel a DNA test of that individual for comparison with the DNA evidence collected from the victim or at the crime scene.