Researchers trying to learn what killed the first person to receive a heart transplant from a pig have discovered the organ harbored an animal virus but cannot yet say if it played any role in the man’s death. A Maryland man, 57-year-old David Bennett Sr., died in March, two months after the groundbreaking experimental transplant. University of Maryland doctors said Thursday they found an unwelcome surprise — viral DNA inside the pig heart. They did not find signs that this bug, called porcine cytomegalovirus, was causing an active infection.

A Maryland hospital is defending its decision to transplant a pig’s heart into a dying man following reports that the patient had a criminal past, saying his eligibility was “based solely on his medical records.”