An investigation into the construction of Zimmer began in 1978. A worker charged that a welding contractor did defective work below nuclear safety standards. CG&E's lawyers placed blame on the contractor for not picking up on the defective work. In 1980 the Chicago Sun-Times reported accusations of a cover-up by CG&E of time-card falsifications and the installation of defective piping in one of the plant's safety systems. As a result, the Nuclear Regulatory Commission (NRC) fined CG&E $200,000 for a faulty quality assurance program in 1981. The constructor was Kaiser Engineering whereas the primary owner, CG&E, did its own procurement, awarding contracts for equipment, and quality assurance requirements. The NRC, under pressure from the Government Accountability Project (GAP) to reopen the investigation, eventually ordered work on the nuclear reactor to halt in 1982. The Federal Bureau of Investigation (FBI) launched an investigation into the plant's safety records and harassment of inspectors.
