NASHVILLE, Tenn. – With federal officials having identified the man believed to be behind Nashville's Christmas Day bombing, authorities now turn to the monumental task of piecing together the motive behind the explosion that severely damaged dozens of downtown buildings and injured three people.While officials on Sunday named Anthony Quinn Warner, 63, as the man behind the mysterious explosion in which he was killed, the motive has remained elusive. “These answers won't come quickly and will still require a lot of our team's efforts," FBI Special Agent Doug Korneski said at a Sunday news conference.