Complex sentence: At least 500 people, including children, were in an upstairs room enjoying the festivities, when an unknown person, assumed by the miners to be a mining company "goon," falsely shouted "fire".
Split into simpler sentences: At least 500 people, including children, were in an upstairs room enjoying the festivities. An unknown person, assumed by the miners to be a mining company "goon," falsely shouted "fire".