LAS VEGAS — President Clinton conceded Sunday that the administration mistakenly obtained the FBI files of more than 300 people, including many top Republicans, in late 1993 as the result of a "completely honest bureaucratic snafu" involving security clearances. His chief of staff, Leon Panetta, told reporters that "obviously a mistake was made" and apologized to the people whose FBI files wound up at the White House. Clinton did not apologize directly, but said "I completely support" what Panetta said about the affair.