NASHVILLE, Tennessee, March 27 (Reuters) - A 28-year-old woman armed with several guns on Monday fatally shot three children and three adult staffers at a private Christian school she once attended in Tennessee's capital city before police killed her, authorities said. There was no immediate official word on a possible motive for the gun violence, which unfolded on a warm, sunny spring morning not long after classes had begun at The Covenant School, whose students consist mostly of elementary school-age children. Nashville Mayor John Cooper expressed sympathy for the victims and wrote on social media that his city "joined the dreaded, long list of communities to experience a school shooting