Paper Trails

In all the news stories I've read through for the roundups, I don't think I've ever seen this happen. This is an article about a series of Georgia homes that were busted for having indoor grow rooms.

The current series of marijuana raids began after authorities in Fayette County received a tip from DEA agents in Miami, said Capt. Mike Pruitt of the Fayette County Sheriff's Department. Investigators arrested the owner of a hydroponics store, which focuses on soil-less growing, in Fayetteville and the paper trail led them to grow houses scattered across metro Atlanta.

I'm guessing the paper trail was information from credit card transactions? What does that mean from a legal standpoint? Does anyone know? Is that common in drug investigations?