Raises a lot of questions about privacy issues and these ancestry sites.
More is coming out about this and it highlights even more how modern society is just prepared to put all of its personal data out there. In this case, first an FBI lawyer checked the plan they had proposed and said it was legal. From that, the investigator from the DA's office didn't need to get a warrant and have to trawl through private files/information - he simply joined the GEDmatch website under a false profile and pseudonym, submitted the DNA they had from a crime scene from 1980 and then the company did a search, as any of us can do, to look for "lost" relatives and returned the results. From this they got some hits and then they worked through this to get their suspect.
You can imagine that right now law enforcement all over the place are submitting DNA to these companies for unsolved cases - I certainly would if I was a detective.