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 — including Bennett’s transplant, which initially appeared successful, and the attachment of a pig kidney to a brain-dead patient on a ventilator. The kidney and heart came from a company called Revivicor, which has genetically altered pigs to inactivate genes that could trigger a human body to reject an organ.

Although it’s still too early to say if organ rejection played a role in Bennett’s death, researchers involved in xenotransplantation procedures have stressed that early, positive outcomes don’t necessarily mean long-term success. Even with more routine and well-matched human organ transplants, rejection can happen years after the operation.

Xenotransplantation could be a major innovation, but should develop with care and transparency, kidney transplant expert Peter Reese, a researcher at the University of Pennsylvania, tweeted in response to the news of Benentt’s death: “The right road is through oversight, deliberate patient selection, transparency, robust ethics input, peer review, and humility from all.”