An internal Justice Department investigation into the F.B.I. crime laboratory has uncovered numerous complaints by laboratory employees about the handling of forensic evidence in one of the Government's most important criminal cases, against two men charged with the bombing of the Oklahoma City Federal Building in April 1995. The criticism of the F.B.I.'s conduct emerged in a series of interviews conducted by investigators from the inspector general's office. Laboratory examiners in Oklahoma shipped critical items to the laboratory, like the faded black jeans worn by Timothy J. McVeigh ... in a brown paper sack instead of a sealed plastic evidence bags, one employee said. At one point, visitors to the laboratory placed travel cases that were potentially contaminated with residue of the explosion in an area where bomb debris had been stored awaiting testing, another employee said. As a result, none of the material could be tested. Mr. Williams, the chief laboratory examiner in Oklahoma City, was transferred from his job and was withdrawn as a prosecution witness in Oklahoma City. Mr. Williams had been responsible for conclusions about several major issues in the case, like the size of the bomb that tore the front off the Federal Building. His opinion that the bomb contained 4,000 pounds of ammonium nitrate was an estimate based not on scientific studies but in part on searches of the defendants' houses. Two laboratory workers said Mr. Williams had changed their dictated reports, in violation of F.B.I. policy.