"However, the prominence of Denver was marred by incidents, such as a disastrous leakage from the nearby Rocky Mountain Arsenal, which created a poisonous cloud that hit Denver, resulting in a high number of casualties."
"Then, an outbreak of the New Plague hit the city. Rioters burned down large parts of the city in their fear of contamination and anger at their treatment. "
Those have something to do with it I bet. The Plague or the leakage and poisonous cloud. The Poisonous cloud almost reminds me of the siera madre with the poisonous fumes and stuff.