Federal investigators believe a mid-level federal manager who single-handedly pulled off the largest embezzlement case in government history probably took more money than first thought. And because records were routinely destroyed, the full extent of the embezzlement may never be known. Sources close to the investigation of William Burns, a financial manager for the Agency for International Development, said Monday he is cooperating with investigators from the Justice Department and the AID inspector general. Burns, 48, pleaded guilty Aug. 11 in U.S. District Court to three felony counts and admitted the theft of $1.2 million during a six-year period. But the sources said investigators have discovered Burns' embezzlement of government funds began in the fall of 1980, not in 1982 as laid out in his guilty plea.