An Indiana man faces federal charges of child abuse after forcing his three young grandsons on dangerous Grand Canyon hikes, federal authorities said.

The boys are 8, 9 and 12 years old.

The Indianapolis man forced his grandsons to hike for miles without rest, food or water, according to a criminal complaint filed in U.S. District Court in the District of Arizona.

The complaint alleges that he abused the boys by whipping, choking, pinching, hitting, squeezing, pushing and kicking them with a metal-toed boot.

The boys reportedly told investigators that they became extremely ill from the heat and vomited several times throughout the hikes.

On the first hike, a park ranger saw the group at Indian Garden, according to the criminal complaint. Authorities believe the grandfather had forced the boys to hike over 7 miles that day.

The grandfather wanted to continue the hike despite the boys' conditions. The ranger refused to allow him to do so.

At one point, the grandfather forced the boys to hike 19 miles at the same time the temperature reached 108 degreess at Phantom Ranch along the Colorado River.

The boys were denied water and had to "sneak" sips from the river, according to federal documents.

After the group was identified on their second hike, the children were rescued by rangers and put in the care of Arizona's Child Protective Services.

The boys' lips had been sunburned, and medical exams indicated they suffered from numerous injuries, ranging from cuts and bruises to a potentially fatal heat stroke, according to the federal complaint.

The boys reportedly indicated previous abuse by their grandfather while on other trips this summer. Evidence of this was found on their bodies by a nurse, the complaint says.

The grandfather is in custody at the Coconino County Jail.

To protect the children's identities, The Arizona Republic is withholding the name of the grandfather.