Two adult chimpanzees that viciously attacked a U.S. student at a primate sanctuary in South Africa were defending their territory and will be allowed to live, the lead government investigator said Tuesday.

Conservationist Dries Pienaar blamed human error for Thursday’s attack.

But one of the sanctuary managers, Eugene Cussons, said he did not blame Andrew F. Oberle for crossing between two safety fences to retrieve a rock that the chimps were in the habit of throwing at tourists.

Oberle was in critical condition and in a medically induced coma in the hospital by Monday night. On Tuesday, doctors refused to describe his condition saying the family, which has arrived from the United States, is traumatized and asking for privacy.

Pienaar told The Associated Press that the chimps tore off one of Oberle’s testicles and some fingers from one hand as well as mauling his head. This was “to my astonishment, I couldn’t believe it because I know those chimps personally,” he said.

He said he found no negligence on the part of the Jane Goodall Institute’s Chimpanzee Eden SA in eastern South Africa.

“The only thing that happened is Andrew stepped over the small barrier fence and went right up to the electric fence,” he said. “We all know that they are tame chimps, but he shouldn’t have done that, he’s a researcher, he’s supposed to read the body language.”

Oberle was leading a group of tourists at the time. The visitors were 10 meters (33 feet) from the second fence, as required by safety rules. After Oberle stepped over the first fence, the chimps dragged him under the electric fence, then out into a public area where they continued to attack him, Cussons said.

Cussons said he was happy that Pienaar found the chimps were involved in territorial defense and would not therefore be killed or punished.

He said he was forced to shoot one of the chimps, but not mortally, after he and a ranger failed to scare the animals into releasing Oberle. When they drove a car at them, chimp Nikki jumped onto the front and smashed the windshield, causing Cussons to fire.

Nikki, aged about 16, was wounded in the abdomen and is being treated at the Johannesburg Zoo.

The other attacker, Amadeus, in its 20s, is on lockdown with its family at the sanctuary.

Pienaar, who has worked as a conservationist for 33 years, said he condoned the shooting, a last option under protocols that recommend first shock treatment or pepper sprays.

“Other than that I’m happy with things,” Pienaar said. “I’m not having the chimps put down. I don’t think there’s reason for that.”

Oberle is a post-graduate student of anthropology and primate behavior at the University of Texas at San Antonio. It was his second trip to study at the South African institute, which takes in orphaned and abused chimpanzees.