The lead South African government investigator says two chimpanzees who viciously attacked a U.S. student will be allowed to live.

Conservationist Dries Pienaar blames human error for the attack last Thursday that has left 26-year-old Andrew F. Oberle in critical condition.

Pienaar told The Associated Press on Tuesday he found no negligence by the Jane Goodall Institute Chimpanzee Eden sanctuary in eastern South Africa.

Sanctuary managing member Eugene Cussons said he did not blame Oberle for crossing between two safety fences to try to retrieve a rock that the chimpanzees would use to stone tourists.

The chimps dragged Oberle under an electric fence and Pienaar said they tore off some fingers of one hand, among other injuries.

Oberle is studying anthropology at the University of Texas at San Antonio, and was researching at the institute at the time of the attack.