A 13-year-old boy among on a hayride in southeastern North Dakota was run over by one of two flatbed trailers being pulled and seriously hurt, authorities said Sunday.

The incident occurred early Saturday afternoon in Hankinson, a city about 20 miles west of the Minnesota border, according to the North Dakota Highway Patrol.

The boy was first put in an ambulance for transport to Sanford Health in Fargo, but he was transferred while en route into a medical helicopter to speed up the 65-mile trip to the hospital.

There were about 25 riders on the two flatbed trailers behind a tractor, which was traveling roughly 5 miles per hour, according to the patrol. Authorities have not explained what led to the boy being run over by the second trailer.

The tractor’s driver, 59-year-old Donald Buckhaus, of Hankinson, was not hurt, nor were any of the other riders, the patrol added.