In years past on the Delphi threads, this suggestion of a "DNA dragnet" has come up many times. Yes, there are a few other cases where it has been done by LE. But this investigative tactic has also been legally challenged in the US where we have constitutional protections over being compelled or, if "voluntarily asked," coerced to give DNA. I believe that investigators in the Delphi case, where they appear to have been so careful to do things strictly by the book (legally speaking), they were not willing to risk getting a lead this way that could ultimate jeopardize their case. I mean, there's a reason that these dragnets are rare and police don't do them for every single older or cold case. Just my opinion.