AZTEC, N.M. (AP)>> A 21-year-old gunman who disguised himself as a student to get into a New Mexico high school where he killed two students had caught the attention of U.S. investigators more than a year ago, authorities said Friday.William Atchison, a former student at small-town Aztec High School, had legally purchased a handgun at a local store a month ago and planned the attack, authorities said.