WASHINGTON (AP) The U.S. ambassador to Israel is under FBI investigation for suspected security violations, closing a critical channel to Israeli Prime Minister Ehud Barak in the midst of difficult peace negotiations. Apparently the first American ambassador stripped of his security clearances, Martin Indyk is denied access to classified documents and even discussions with Barak or other Middle East figures, a senior State Department official said Saturday. By their nature, such conversations are considered classified. P