The second person to receive a transplanted heart from a pig has died 40 days after the highly experimental surgery. Navy veteran Lawrence Faucette, 58, was dying from heart failure and couldn’t be approved for a human donor because of other medical conditions. He agreed to take part in the experiment, known as xenotransplantation because it could potentially help others.

The pig’s heart was genetically modified to remove the genes that cause rejection by humans’ immune systems. This was an essential step because previous attempts at xenotransplantation, in which animal organs are implanted into humans, have failed. In one experiment, a baboon’s heart was transplanted into another baboon and survived for only a few weeks, while a pig’s heart implanted into a mouse lasted for two months.

Doctors at the University of Maryland School of Medicine operated on September 20 in an eight-hour surgery. Faucette, who had been on a waiting list for a human heart, seemed healthy for the first month, and doctors were encouraged when a blood test showed his immune system was not attacking the new heart. He began physical therapy and could watch the Super Bowl on TV. “He fought to the end,” his wife Ann said in a statement released by the hospital.

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But the 58-year-old’s condition quickly deteriorated. His lungs and kidneys were failing, and his blood pressure was dangerously high. Doctors tried to restart his heart using a medication that temporarily slowed the patient’s blood flow, but it was too late. He died on Monday.

Scientists are still determining what caused the failure. They speculate that the transplanted pig heart was infected with a virus called porcine cytomegalovirus, which can kill both the pig and the human who receives it. However, the virus found in the donor pig was not expected to infect humans, and no disease cases have been linked to it in the United States.

Doctors will now try to figure out what went wrong in Bennett’s case with the hope of improving future procedures. They may use gene editing to make pig hearts even more resistant to rejection by removing the genes that trigger it, and they may seek the permission of the US Food and Drug Administration for another attempt at a transplant with a modified pig heart. It needs to be clarified whether the next attempt will work as well as the first, but there is no question that it’s needed to reduce the waiting list for human donors and save many more lives. A company called eGenesis and another company, United Therapeutics, are working on developing such a modified heart. They hope to have clinical trials running in a year or two. The CDC reports that there are more than 100,000 people on the waitlist for a heart transplant in the US. That’s down from a peak of about 175,000 in 1990, but it’s still far too high for people who need one.