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.

But a major worry about animal-to-human transplants is the risk that it could introduce new kinds of infections to people.

Because some viruses are “latent,” meaning they lurk without causing disease, “it could be a hitchhiker,” Dr. Bartley Griffith, the surgeon who performed Bennett’s transplant, told The Associated Press.

Still, development is under way of more sophisticated tests to “make sure that we don’t miss these kinds of viruses,” added Dr. Muhammad Mohiuddin, scientific director of the university’s xenotransplant program.