Andrew Oberle, a 26-year-old studying anthropology at the University of Texas at San Antonio, was leading a tour at the Jane Goodall Institute Chimpanzee Eden near Johannesburg Thursday when reports say he left the group and crossed one of two fences separating him from the animals.
When he neared the second fence, which was electrified, two chimpanzees reached underneath and pulled him by his feet into their enclosure.
Oberle was attacked by the animals and dragged half a mile before armed guards and staff members were able to enter the enclosure and rescue him.
"The chimps were still out there. ... He was curled up in a little ball," Lloyd Krause, an ambulance service manager who entered the enclosure to remove Oberle, told ABC News today.
Oberle, who suffered from severe bite wounds, underwent surgery at the Mediclinic hospital in Nelspruit, 180 miles from Johannesburg, according to The Associated Press. He is now in critical condition.
In a statement, the Goodall Institute said, "This is a terrible tragedy that should never have happened. ... All our thoughts and prayers are with this young man and his family."
The university said in a statement today that "our hearts go out to Andrew and his family."
His mother, Mary Flint of St. Louis, told the AP today that he'd been in South Africa since May and that he'd passionate about chimpanzees since the seventh grade when he'd seen a film on Goodall.
David Oosthuizen, the institute's executive director, said the sanctuary had been closed temporarily after the attack. Edwin Jay, the institute's chairman, said the two chimpanzees involved in the mauling would be returned to the enclosure after an investigation into the incident.
The Associated Press contributed to this story.