Thanks. This is helpful.
Just to clarify my question, I wasn't really wondering about excluding or eliminating people as suspects. It's more about, does this mean that they definitively HAVE found unknown DNA already, and now need to match it to someone? Or could be more that they have now built up what they believe is a fairly complete list of people who worked in the house, and they are now going down that list to collect DNA (and maybe other info from them), so that if and when they find unknown DNA, they have a "library" so to speak of samples to try to match it against.
Basically, I'm just questioning the premise that the DNA testing proves they have already found something in the house, as the expert said. I hope it does, because I want them to figure it out. And if you, and other experienced people here, tell me that's what it means, I will believe you. Just as a lay-person, absent further confirmation, it didn't sway me. It seemed reasonable to proactively gather DNA samples from anyone who may be around, so they don't have to scramble to track people down and do it if they do find something to check it against.