MORTON — Fearing for his safety, a teenager jumped through the open door of a carnival ride that began to move Thursday night at the Morton Pumpkin Festival.

Police are investigating the mishap on The Zipper, which left the teen with a hurt shoulder, said Morton Police Chief Craig Hilliard. A second teen waiting to get into The Zipper leaped out of the way of the moving ride but was not injured, Hilliard said.

The carnival operator, Big M Amusements of New Holland, could not be reached for comment Friday. Big M was the carnival operator at the same festival two years ago when the door of another ride opened.

The Zipper involves a rotating oval boom from which suspend a dozen or so cage-like cars that spin top to bottom. Big M's Zipper is operated by either two or three employees: One runs the ride, while either one or two other workers help riders step out of and into the cages.

Each cage has a door that swings open. Before the ride moves, the door is to be latched shut by a worker, Hilliard said.

About 8:15 p.m. Thursday, two teen boys prepared to enter a cage after a worker had opened the door. One teen stepped inside and sat atop a bench, and the door remained open. Before the second teen could enter the open cage, the ride began to lift upward, Hilliard said.

Spooked, the teen inside the cage scrambled out the doorway and jumped to the ground, Hililard said.

"He was worried how high he was going to go up with the door open." Hilliard said.

The teen struck the ground with his shoulder, prompting a call to on-site medical personnel, Hilliard said. The injury was not serious, although the teen had a sore shoulder Friday, Hilliard said.

As the ride moved, the second teen leaped sideways. He was not hurt, Hilliard said.

Police are trying to determine why the ride began to move. Police have been unable to locate the worker who was supposed to have locked the cage, Hilliard said. That worker also might have been struck by the ride, Hilliard said.

However, Hilliard said human error is probably to blame for the mishap: "It was not mechanical."

With the mechanics deemed sound Thursday night, The Zipper was allowed to resume operations. Police continue to investigate the matter, which will likely be turned over the state's Amusement Ride and Attraction Safety Board, a division of the Illinois Department of Labor.

After the same festival in 2016, the board examined fest-goers' complaints about the Rock-O-Plane, which is like a smaller version of The Zipper. As the ride whirled, the door on a cage with a young girl inside popped open.

For the next minute or so, as the ride slowed so an attendant could grab the cage, the girl went almost a full revolution without a safety door in place. She was not hurt.

"The department did investigate the Rock-O-Plane after the 2016 incident and Big M took corrective action," said Ben Noble, spokesman for the labor department.

Noble had no additional information available about that case.

PHIL LUCIANO is a Journal Star columnist. He can be reached at pluciano@pjstar.com, facebook.com/philluciano and (309) 686-3155. Follow him on Twitter.com/LucianoPhil.