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GRANBY, Colo. — The fall from a chairlift at Ski Granby Ranch last month that left a mother dead and her two children injured happened during a mechanical problem, according to a document released Monday.

The Colorado Passenger Tramway Safety Board released details related to the Quick Draw Express lift incident from Dec. 29. Those details were included in an agreement that allows the lift to begin operating again.

The 25-foot fall killed 40-year-old Kelly Huber of San Antonio who was pronounced dead at a Granby emergency room.

Huber’s 12-year-old daughter was treated and released at a hospital in Granby and her 9-year-old daughter was treated at Children’s Hospital Colorado in Aurora.

According to the document, the chair they were riding in struck a support tower, and Huber and her children were thrown from the chair.

Engineers reported that issues within the chairlift’s electrical drive and control system created a “rare dynamic event.”

Environmental factors, weather and the actions of the victims were cleared as having anything to do with the incident.

The ski area was authorized to reopen the lift with specific conditions, including reducing the speed of the lift. It will reopen Tuesday.

“Granby Ranch would again like to extend its condolences to the family of our guest who lost her life, and her family members who were injured in this tragic event,” the resort said in a statement.

“Granby Ranch places the highest value on the safety of our guests and the safe operation of all lifts and equipment at the resort. We would like to thank all of our guests, homeowners and team members for their supportive words and thoughts during this very difficult time.”

Huber suffered a traumatic rupture of her aorta, according to an autopsy. She also suffered other blunt force injuries.