Potentially fatal letter bombs arrived at the homes of British Prime Minister Margaret Thatcher's press secretary and two senior civil servants, but police specialists disarmed them, Scotland Yard reported. The British Broadcasting Corp. said the outlawed Irish Republican Army claimed responsibility for the attempts. Letter bombs were received by the families of Bernard Ingham, Thatcher's press spokesman, and Brian Unwin, head of the Economic Secretariat in the Cabinet Office. Neither official was at home when the devices came in the mail. The third letter bomb was delivered to an unidentified senior civil servant's home in North London.