CARRABASSETT VALLEY, Maine (CBS) — Investigators believe a mechanical failure caused one of Sugarloaf’s ski lifts to roll backwards Saturday injuring seven.

A preliminary investigation conducted by resort personnel and the State of Maine Board of Elevators and Tramways found that one of two gearboxes connecting the lift’s electric motor to its drive bullwheel had stopped working.

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The chairlift began moving in reverse Saturday, prompting some skiers to jump. Sugarloaf says four people were transported from the resort in ambulances.

A backup brake system was able to slow down the speed of the rollback.

Three adults were treated at Franklin Memorial Hospital in Farmington with non-life threatening injuries, according to a hospital spokesperson. Two patients were later discharged from the hospital. One was transferred to Central Maine Medical Center.

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The gearbox had undergone routine preventive maintenance and passed inspection just one day before the incident, according to the resort.

“The cause of the gearbox failure and the failure of the drop dog to deploy as designed both remain under investigation,” Ethan Austin, Sugarloaf’s Director of Marketing said in a public statement Sunday. “It is likely that determining the root cause of these failures will require extensive analysis.”

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The lift remains closed pending further investigation.