FLAGSTAFF, Ariz. -- An Indiana man forced his three young grandsons to hike 18 miles in triple-digit heat at the Grand Canyon and denied them food and water, authorities said.

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 on a popular trail at the canyon's South Rim last month.

On the latest hike over the weekend down the Bright Angel Trail, temperatures reached 108 degrees at Phantom Ranch along the Colorado River. A ranger spotted the group with binoculars on the trail and saw Carlson shoving the oldest boy and whipping him with a rolled-up T-shirt, authorities said.

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.

Authorities said Carlson tortured and beat the boys, and instructed them to lie to park rangers about any injuries. Rangers and passers-by noted the alleged abuse by Carlson, according to court documents.

The boys said Carlson also forced their fingers down their throats, making them vomit.

Rangers fed and hydrated the boys inside an ambulance 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.

The Bright Angel Trail can be deceiving. It starts at the top of the canyon at around 7,000 feet in elevation and drops to 2,400 feet by the river, and the temperature varies widely. The Park Service advises hikers not to make the trip to the river and back in one day. Warning signs are posted at the trailhead and along the trail.

Defense attorney Luke Mulligan questioned the children's statements, saying it seemed improbable that they could have completed the hike without food and water. He also said the rangers could have removed the children from the canyon had they believed the children were at risk of serious injury or death.

A federal magistrate found probable cause for allegations of child abuse and determined that Carlson, 45, was a flight risk and a danger to children.