David Bennett, 57, who received a pig heart in January in place of his own failing one, died Tuesday. It's not yet clear precisely what caused Bennett's death, according to the University of Maryland Medical Center, where he received the transplant Jan. 7 and had been recovering since. He began deteriorating in recent days and the hospital announced his death Wednesday. Bennett was the first patient ever to receive an animal organ genetically modified to prevent rejection in a person. There was no obvious cause identified at the time of his death, hospital spokeswoman Deborah Kotz said, and researchers plan to conduct a thorough review of his death and publish the results in a peer-reviewed journal. The investigative surgery, although it only added two months to Bennett's life, informed the field of xenotransplantation, which hopes to help solve the human organ shortage. Read more about the procedure and transplant through our link in bio.