Tip of my tongue, a book I think I read in the early 2000s in English, in the United States. The killer comes up as a suspect early in the investigation, but is discounted after the investigators learn that his body was found, and identified via dental records, in the woods. I don't recall if it was thought to be accident, suicide, or murder, but the body is decayed and scavenged enough that the dental records were necessary. Ultimately, it turns out that the killer faked that death scene, I think leaving one of his victims in the woods and changing the records at the dentist so that the victim's images were left in place of his own. Unfortunately, I've forgotten most of the details of his actual kills, although I do remember the killer as male, relatively young (20s or 30s), and I think as a former computer programmer.
The bit that's really sticking with me is that, near the end of the book, the protagonists stumble on a massive bit of government surveillance (which was considered to be something that, if revealed, would cause riots, but kind of sounds prosaic now in the current environment) where they're able to log onto a computer system and track a database of every shopping transaction, toll booth, etc, indexed by an identity. I specifically remember that the menus were an old-style text interface with the protagonists navigating them by typing in the corresponding number for what they were searching, and that one of the bits of data they come up with is where the killer paid for a toll, and his face was captured by one of the cameras.