The Mine Safety and Health Administration (MSHA) (defendant) maintains regulations—known as Part 50 regulations—that cover the notification, investigation, reports and records of accidents, injuries, illnesses, employment, and coal production in mines. Under these regulations, which were adopted via notice and comment rulemaking, mine operators must report occupational illnesses that occur within a mine to MSHA within ten days of the illness occurring or within ten days of it being diagnosed. The agency issued three Program Policy Letters (PPLs) regarding the interpretation of when x-ray results constituted a diagnosis of one of the lung diseases covered by the Part 50 regulations. The PPLs were issued without notice and comment and were not published in the Federal Register, but they were distributed to all mine operators and other relevant parties. The American Mining Congress (plaintiff) argued that the Administrative Procedure Act (APA) required notice and comment prior to the promulgation of the PPLs. Petitions for review were brought the federal court of appeals, which considered whether the PPLs were interpretive rules under the APA.