The first patient to have a heart transplant with an organ from a genetically modified pig died Tuesday, The New York Times reported. David Bennett Sr., who had severe heart disease, received the heart in early January. He was 57.

It’s not clear exactly why Bennett died, a spokesperson from the University of Maryland Medical Center, where the procedure was performed, told The New York Times. Physicians will do a full evaluation and publish their results.

Bennett’s death shows the challenges of animal-to-human transplants, called xenotransplantation. Advocates of the technique imagine a steady supply of animal organs could help treat the thousands of people on waitlists for organ transplants. Researchers and surgeons have shared a wave of successes in recent months