David Bennett, the first human to successfully undergo a heart transplant involving a genetically modified pig heart, has died, according to The New York Times. He was 57. It’s unclear if his body rejected the organ doctors implanted in January. “There was no obvious cause identified at the time of his death,” a spokesperson for the University of Maryland School of Medicine, the hospital that performed the procedure, told the outlet. Physicians plan to carry out a full evaluation before publishing their findings in a peer-reviewed journal.

When Bennett’s transplant was first announced, doctors treated the news with cautious optimism. And for a time, it looked like that feeling was warranted. Not only did Bennett’s body not immediately reject the organ, but he was also able to take part in physical therapy and spend time with his family. And while he was never discharged from the hospital, he did survive two months with the genetically modified organ beating in place of his human heart.

Even if doctors determine the cause of death was organ rejection, that’s no small milestone. Stephanie Fae Beauclair, one of the most famous patients to undergo a xenotransplantation procedure, survived for 21 days before her body rejected her adopted baboon heart. Part of the reason doctors were hopeful the procedure would work is that there’s a dire organ shortage in the US and many others parts of the world. According to the Health Resources and Services Administration, about 17 Americans die every day waiting for an organ transplant.