(KAIR)--A Leavenworth man is dead, killed Thursday afternoon while riding a motoped on Atchison's River Road.

Atchison County Sheriff Jack Laurie says 56-year-old Mark Randolph Forman was riding the motorized bicycle southbound on River Road, about a mile south of 244th Road, when the fatal wreck happened.

According to Laurie, Forman came off a bridge while traveling between 15 to 20 mph, struck loose gravel, and veered off the road. The bike's front tire struck a very small indentation on the side of the road, which caused the bike to throw him off, likely over the handlebars. The investigation indicates Forman, who was wearing a helmet, may have landed on his head, possibly suffering a major neck injury.

Laurie says an autopsy will be conducted to confirm the exact cause of Forman's death.

The deadly accident happened shortly before 2:30 Thursday afternoon.