Three people, including two children, were rushed to a Colorado hospital after colliding on a zip line. 

A child and an adult were listed in critical condition as of Sunday night, authorities said. The other child reportedly had less severe injuries but was also taken to the hospital for treatment. 

The incident happened at Philip S. Miller Park in Castle Rock, about 40 miles south of Denver. 

An investigation is underway to determine what caused the collision.

Castle Rock Fire and Rescue said that when medics arrived, the individuals involved in the collision were no longer on the zip line and were on one of the park’s platforms. 

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The zip line tours at the park are operated by the EDGE. According to its website, the park contains 10 zip lines that span more than 2 total miles and stand as high as 110 feet off the ground.

Participants can reportedly reach speeds up to 50 mph, according to the site.

This story was originally published by Landon Haaf at Scripps News Denver. 

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