FLAGSTAFF, Ariz. (AP) — An Indiana man forced his young grandsons for miles along a sun-baked Grand Canyon hiking trail and denied them water and food, saying later he wanted to toughen them up, according to authorities.

Christopher Carlson, of Indianapolis, remained jailed Thursday on six counts of child abuse. The boys, ages 12, 9 and 8, told investigators that they had been hit, pushed, choked, pinched and squeezed during trips into the canyon from the South Rim last month.

A ranger with binoculars spotted the group on its most recent hike — a trek last weekend on the popular Bright Angel Trail and saw Carlson shoving the oldest boy and whipping him with a rolled-up T-shirt, authorities said.

Rangers fed and hydrated the boys and they were placed in the care of child protective services. One boy had symptoms of heat stroke, while the other two exhibited signs of heat exhaustion and dehydration.

National Park Service Special Agent Chris Smith testified that Carlson told authorities that the boys had been overweight and that he thought the hike would get them into shape.

“He told me that he loved his grandchildren very much, but at the same time there were tough people in the world and his grandchildren needed to be tough as well,” Smith said.