The man who received a first-of-its-kind heart transplant from a pig died two months later of heart failure, according to a study from the University of Maryland School of Medicine that still failed to get to the bottom of the tragic turn of events. David Bennett, 57, was able to walk around and spend time with family after the groundbreaking transplant in January. “We consider that he lived through the surgery the first win,” study co-author Muhammad M. Mohiuddin said, according to The Washington Post. “When he seemed to be recovering and doing well for two months, we really thought that was a huge success. If we could have identified the reason his heart gave out suddenly, he might have walked out of the hospital.” Mohiuddin said researchers are still trying to figure out “what went wrong; we don’t have a single answer.” Separately, Bennett’s surgeons said after his death that the heart was infected with a porcine virus.