>> The fact that the government of North Dakota is still combing the scene for DNA leads to prosecute additional protestors 3 years later

I doubt this DNA was combed from the scene three years later. This was a cigarette that was taken three years ago and has been in an evidence locker for years. Finally, today, an arrest is being made. We don't know whether it took three years to do the testing or whether they knew of the match years ago and only now decided to arrest. Three years is a common statute of limitations limit. They could have been working on this from day one and now, with a SOL looming, they have decided to move forwards.

Or maybe the DNA is the announced link and they had an identification by other means. Maybe by relying upon the cigarette they aren't announcing that they have been talking to witnesses/informers. The cigarette is enough to get the warrant, without the need to divulge the existence of informers just yet.