The daughters of late country music legend Johnny Cash have launched an investigation into why they never received royalties from his hit, RING OF FIRE. Rosanne Cash and her sisters Kathleen, Cindy and Tara - daughters by Cash's first wife, Vivian - claim not to have received money - estimated to be millions of dollars - from publishing rights to the song, and have hired California lawyer Eric Bjorgum, an intellectual property expert, to investigate the matter. Bjorgum tells New York Daily News, "His only son (John) has received the lion's share (from Ring of Fire)". John Cash is the music legend's only child by June Carter Cash. Cash and June Carter Cash are said to have shared the copyright on Ring of Fire with songwriter Merle Kilgore. In her book I Walked the Line, Vivian Cash writes Johnny told her, "I'm going to give June half credit on a song I just wrote called 'Ring of Fire.' When she asked why, he answered, 'She needs the money and I feel sorry for her.'" A lawyer for the Cash estate tells, "We find it noteworthy that their authorship has been unchallenged for over 40 years, and was only questioned after the deaths of Kilgore and the Cashes." Cash died in 2003 after suffering complications from diabetes.