Six rhinos were introduced to Zakouma National Park under a joint initiative combining Chad, South Africa and the NGO, which successfully operates a number of wildlife parks in Africa.
They were put on a 4,800km flight from South Africa, accompanied by a team of vets.
Two were found dead last month.
Post-mortems and tests on blood, tissue and faeces have been sent to a lab in South Africa, African Parks said in the press release. So far, there is no evidence of infectious disease or plant toxicity as a cause of death, it said.
Blood samples showed exposure to trypanosomes – a parasite transmitted by tsetse flies – but this is not suspected to be the cause of the tragedy, it said.
“Low fat reserves suggest that maladaptation by the rhinos to their new environment is the likely underlying cause, although tests to be undertaken on brain and spinal fluid may shed additional light on the exact cause of deaths,” it added.