Edit: I used to help Google fund researchers like Joe Derisi and others who develop technology to do this, and some of the people I worked with in my academic career are quite good at identifying serial killers from 30 year old DNA. If you're downvoting because you think I'm making this up, you're wrong. If you're downvoting because you don't think large-scale individual detection using genetic sampling of the environment is possible, you're wrong. If you're downvoting because you think you couldn't do a whole genome sequence of an individual using a sample collected in the wild, you're wrong. If you're downvoting because you think this is a terrible idea (morally, ethically), that's fine but I didn't say anything about my own moral or ethical beliefs about this.

It's simply factually correct to say that large-scale individual sample collection (at order tens of thousands, if not hundreds of thousands of individuals in a country the size of the US) is possible. All the technology is there to do this.