An American student is in critical condition after undergoing two operations after chimpanzees tore apart his body in front of tourists at a South African animal sanctuary, a hospital spokeswoman told CNN on Tuesday.

Andrew Oberle, a primatology student from University of Texas at San Antonio, was being treated at a Johannesburg hospital after two chimps attacked him Thursday, spokeswoman Robyn Baard said.

Oberle had been at the Jane Goodall Institute's Chimp Eden since May, according to Eugene Cussons, the facility's managing director. Oberle was at the sanctuary, near Nelspruit, South Africa, for the second time after training and volunteering there in 2010.  His training included an explanation about "no-go" areas - spaces for animals where people are not supposed to go.

Witnesses to the attack said that Oberle went into a no-go area because he seemed to want to remove a stone close to one of the animals that could have been picked up and thrown around, Cussons told CNN.

Oberle crossed one barrier and approached a second one, which is a main fence with 24 strands of electrical wiring, Cussons said. Two male chimps grabbed Oberle and tried to drag him under the fence, but were not able to yank him into their enclosure.

Cussons said he estimates the attack lasted 15 minutes.

At some point, people tried to stop the chimps, and Cussons shot two rounds in the air to see if that might get them to retreat, he said. One of the chimps then charged at Cussons, he said. Cussons shot that chimp in the abdomen, he told CNN, and it seemed to shriek as a kind of signal to other chimps that there was a more powerful threat present. The chimps then backed off, he said.

Oberle was rescued and transported for medical care.

None of the 13 tourists - most of them from local areas - were harmed, officials said.

The chimp that was shot had an operation at the Johannesburg zoo to repair damage to his small and large intestines.

Hospital spokeswoman Baard declined to discuss the nature of Oberle's wounds. She said the student's parents had requested privacy, adding that they are "quite traumatized."

The sanctuary, which is featured in the Animal Planet program "Escape to Chimp Eden," remains closed and its staff is receiving counseling, executive director David Oosthuizen said.

There are no plans right now to euthanize the chimps involved in the attack, said Dries Pienaar, who is leading the investigation into the incident. He works for a parks agency that makes sure zoos, sanctuaries and breeding projects comply with the law. Pienaar told CNN that his preliminary findings are that human error is to blame, but he cautioned that his investigation is not complete and that he wants to interview Oberle. He hasn't spoken to all of the tourists yet, either.

Chimp Eden was established as a home for rescued chimpanzees. Many of the primates have suffered "horrible injuries and abuse from humans," according to the sanctuary.

Dave Salmoni, an expert in large predators for the television channel Animal Planet, said abused and captive chimpanzees can be particularly dangerous, likening the chimps to troubled prison inmates.

"Now this is a very nice prison, but it's a prison nonetheless," he said Monday. "And that's why you can see a lot of acting out behavior, and in some cases, with chimpanzees, they act out just because they can."

Oberle was passionate about studying chimpanzees, his friend Anthony Reimherr told CNN affiliate KXAN-TV. He said it was "intriguing" to listen to Oberle when he spoke about the animals.

"It's just something that he loved to do, and I think it's something that he'll always continue to do," Reimherr said.