The Rocky Mountain Arsenal (RMA), a few miles northeast of Denver, was originally constructed and operated by the Chemical Corps of the United States Army. Beginning in 1942, the arsenal was the main site at which the Chemical Corps manufactured chemical weapons such as mustard gas, nerve gas, and phosgene. The Army eventually leased part of the 27 mi² (70 km²) plot of land to the Shell Oil Company which produced DDT, dieldrin, chlordane, parathion, aldrin, and other pesticides at the site.

The presence of a chemical weapons plant has long been a source of concern for many Coloradans. In 1968, for example, a group of Denver-area residents complained that nerve gas was being stored in an open pit directly beneath one of the flight paths into Denver's Stapleton International Airport.

Indeed, the Army was well aware of the hazard posed by its RMA products. In 1961, it found...