Federal officials announced yesterday that thousands of environmental safety tests performed at Superfund locations and other hazardous waste sites around the U.S. between 1994 and 1997 will have to be repeated because a testing company falsified results. Federal prosecutors are planning criminal indictments against 13 former employees of Intertek Testing Services, formerly the second-largest tester in the U.S. of toxic materials. An Intertek unit in Texas is suspected of falsifying data on some 59,000 projects involving more than 100,000 different tests conducted on samples of soil, groundwater, and other materials taken from potentially toxic sites. The massive fraud case means that some sites believed to be safe could actually contain carcinogens and other dangerous pollutants, though the feds said that none of the sites retested so far has been found to pose health hazards.