The girl has been identified only as a six-year-old from Colorado Springs who was on vacation with her family, 9News reported.
The exact circumstances of her death are unclear. CBS Denver reported that police radio messages around the time of the accident requested help for "a party that fell out of the shaft ride."
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That detail has not been confirmed by Glenwood Caverns or public officials.
Park employees performed first aid until paramedics and fighters arrived, who pronounced her dead, 9News reported, citing Garfield County's coroner's office.
In a statement sent to Insider, the attraction's General Manager Nancy Heard said: "An investigation that will include state and local authorities, as well as independent engineering and ride inspection experts and our own maintenance team, is under way."
The park was closed Monday and Tuesday.
On its website, Glenwood Caverns describes itself as "America's only mountain-top theme park," a family-run business that features thrill rides and trips into "fairy caves" in the mountains.
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A 2019 video featuring the mine drop ride shows a member of staff demonstrating its safety features, which included two types of seatbelt restraint:
The company's website no longer includes the ride description, but an archived version of the page from May describes it as based around a "ghost story of a mining tragedy from long ago." Riders encounter "ghostly miners" both before and after the drop, it says.
"Out of respect and concern for all parties involved, we will not have further comment until all details have been confirmed," a statement released by the park on Monday said. "Our thoughts and prayers go out to all involved."
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