A U.S. federal trial began Wednesday for an Indiana man accused of forcing his grandsons to hike for kilometres in the Grand Canyon without food or water in brutal August heat.

Investigators have said that Christopher Alan Carlson, of Indianapolis, told them that the boys were overweight and that he thought hiking the Grand Canyon would help 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," National Park Service Special Agent Chris Smith said at the time.

Carlson, who is in his mid-40s, has pleaded not guilty to six counts of child abuse.

Christopher Carlson is accused of forcing his young grandsons to walk for kilometres along a sun-baked Grand Canyon hiking trail while denying them water and food. (Indianapolis Metro Police Dept/Associated Press)

Jury selection in his trial began Wednesday and was expected to wrap up by Thursday. Opening statements from the prosecution and defence will follow.

Judge Frederick Martone started questioning a pool of 56 jurors whether they had heard about details of the case from the news media.

A dozen people said they had heard about it on the news, and one of them said the coverage combined with his own experience with abuse at the hands of his father would affect his ability to be impartial.

Allegations of abuse

Carlson's grandsons — who were 12, 9 and 8 years old at the time — told investigators that Carlson hit, pushed, choked and squeezed them, and forced their fingers down their throats to make them vomit during trips into the Grand Canyon.

They also said they weren't allowed to eat breakfast the morning of Aug. 28 and were given only celery on the hike from the South Rim down to the Colorado River.

A ranger with binoculars spotted the group on what would be the last of the hikes on Aug. 28, when the temperature soared to 108 degrees and a man died on another trail from heat exposure. The ranger reported seeing Carlson shoving the oldest boy and whipping him with a rolled-up T-shirt.

Rangers fed the boys and gave them water after one showed symptoms of heat stroke and the other two had signs of heat exhaustion and dehydration. They were placed in the care of state Child Protective Services.

Defence attorneys have questioned the boys' statements, saying that it seemed improbable that they could have gone on such a hike without food and water.

The boys' mother, Tara Danaher, of Indianapolis, sobbed at a court hearing on Sept. 1 and said her children went on trips with Carlson over the summer, including to Central America and Jamaica. She said she talked with her children throughout the summer and they never expressed any concerns.

The highlight of the latest trip that included the Grand Canyon was supposed to be Disneyland, she said.