The 44 year old was shot in the early hours of Saturday morning (23Nov13) by the owner of the Pit and Barrel bar, Chris Ferrell, who claims he acted in self-defence after an argument broke out.
Mills was admitted to the Vanderbilt University Medical Center in a critical condition, and later died.
Police spokeswoman Kris Mumford says, "We're investigating Chris Ferrell's claims of self-defence."
No arrests have been made.
During his career, Mills toured with the Wayne Mills Band, and stars including Jamey Johnson, Taylor Hicks and Shelton all opened for the group.
Shelton was among the first to pay tribute after hearing of his friend's death, writing in a post on Twitter.com, "Extremely sad to hear about the death of my old friend Wayne Mills... Rest in peace brother." He also sent his condolences to Mills' widow, Carol.
Hicks adds, "My good friend Wayne Mills passed away in Nashville. My thoughts and prayers are with his family. RIP Wayne he was my buddy", while Bentley writes, "Man....totally shocked and saddened to hear about Wayne Mills' death. Honky tonker and true lover of country music. RIP Wayne."
Paul MCDonald adds, "Rest in peace Mr. Wayne Mills. The Wayne Mills Band and I toured many years in the same circle & I always respected and looked up to his craft. He will be missed by many."