This case involves a dispute between Critical Mass Energy Project (P) and the Nuclear Regulatory Commission (D) over access to safety reports prepared by the Institute for Nuclear Power Operations ('INPO') and voluntarily transmitted to D on the condition that the agency will not release the information to other parties without INPO's consent. INPO was formed after Three Mile Island accident to promote safety and reliability in the operation of nuclear power plants. INPO is a nonprofit corporation whose membership includes all operators of nuclear power plants in the United States. P asked D, pursuant to FOIA, to provide it with copies of the INPO reports. D denied the request, finding that they contained confidential commercial information and were therefore protected from disclosure by Exemption 4. P then brought suit in district court challenging D's determination. The court granted D's motion for summary judgment, concluding that the documents were both commercial and confidential and therefore exempt under section 552(b)(4). On appeal, a divided panel reversed and remanded for further proceedings. Applying the National Parks test, the appeals court agreed that the INPO reports were not 'customarily released to the public,' but found insufficient evidence that D's ability to secure the information would be impaired. After remand, INPO intervened in the litigation. Cross-motions for summary judgment were filed, and the district court granted D's motion based on a determination that they had shown that release of the reports would harm the governmental interest in efficiency and cooperation with INPO. The appeal court rejected the district court's rationale but remanded for further findings on the effect of disclosure on the quality of the reports. In response to petitions filed by the D and INPO, the court vacated the opinion in Critical Mass II and ordered that the case be reheard en banc.