For decades, Dr. J. Steve Bynon Jr., a transplant surgeon in Texas, gained accolades and national prominence for his work, including by helping to enforce professional standards in the country’s sprawling organ transplant system.

But officials are now investigating allegations that Dr. Bynon was secretly manipulating a government database to make some of his own patients ineligible to receive new livers, potentially depriving them of lifesaving care.

Memorial Hermann-Texas Medical Center in Houston, where Dr. Bynon oversaw both the liver and kidney transplant programs, abruptly shut down those programs in the past week while looking into the allegations.

On Thursday, the medical center, a teaching hospital affiliated with the University of Texas, said in a statement that a doctor in its liver transplant program had admitted to changing patient records. That effectively denied the transplants, the hospital said. An official with knowledge of the investigation identified the physician as Dr. Bynon, who is employed by the University of Texas Health Science Center at Houston and has had a contract to lead Memorial Hermann’s abdominal transplant program since 2011.

It was not clear what could have motivated Dr. Bynon. Reached by phone on Thursday, he referred questions to UTHealth Houston. He did not confirm he had admitted to altering records.

On Friday, after this article was published online, UTHealth Houston released a statement to news outlets defending Dr. Byon as “an exceptionally talented and caring physician, and a pioneer in abdominal organ transplantation.” The statement said that the survival rates of Dr. Bynon’s patients who received transplants were among the best in the nation. “Our faculty and staff members, including Dr. Bynon, are assisting with the inquiry into Memorial Hermann’s liver transplant program and are committed to addressing and resolving any findings identified by this process,” it said.

Founded in 1925, Memorial Hermann is a major hospital in Houston, but it has a relatively small liver transplant program. Last year, it performed 29 liver transplants, according to federal data, making it one of the smallest programs in Texas.

In recent years, a disproportionate number of Memorial Hermann patients have died while waiting for a liver, data shows. Last year, 14 patients were taken off the center’s waiting list because they either died or became too sick, and its mortality rate for people waiting for a transplant was higher than expected, according to the Scientific Registry of Transplant Recipients, a research group.

This year, as of last month, five patients had died or become too sick to receive a liver transplant, while the hospital had performed three transplants, records show. The investigation is in early stages, and it was unclear if possible changes to the waiting list actually resulted in a patient not receiving a liver. A hospital spokeswoman said the center treated patients who were more severely ill than average.

The U.S. Department of Health and Human Services said in a statement that it was also investigating the allegations. So is the United Network for Organ Sharing, the federal contractor that oversees the country’s organ transplant system.

“We acknowledge the severity of this allegation,” the H.H.S. statement said. “We are working diligently to address this issue with the attention it deserves.”

Officials began investigating after being alerted by a complaint. An analysis then found what the hospital called “irregularities” in how patients were classified on a waiting list for liver transplants. When doctors place a patient on the list, they must identify the types of donors they would consider, including the person’s age and weight.

Hospital officials said they found patients had been listed as accepting only donors with ages and weights that were impossible — for instance, a 300-pound toddler — making them unable to receive any transplant.

Other transplant surgeons said if the list was tampered with, patients would not be aware of changes in their status.

“They’re sitting at home, maybe not traveling, thinking they could get an organ offer any time, but in reality, they’re functionally inactive, and so they’re not going to get that transplant,” said Dr. Sanjay Kulkarni, the vice chair of the ethics committee at the United Network for Organ Sharing. “It’s highly unusual, I’ve never heard of it before, and it’s also highly inappropriate.”

The hospital said in its statement that it did not know how many patients were affected by the changes, or when they began. It said the issues affected only the liver transplant program, but the hospital also closed the kidney transplant program because it was led by the same doctor.

Dr. Bynon, 64, has spent his career in abdominal transplants, and is considered one of the early practitioners of advanced liver transplants. He spent nearly 20 years at the University of Alabama at Birmingham before moving to Texas in 2011.

Some former colleagues described Dr. Bynon as off-putting and arrogant, while others called him talented and dedicated.

“In my experience, everything he did was about the patient,” said Dr. Brendan McGuire, the medical director of liver transplants at that Alabama program, who worked with Dr. Bynon for more than a decade. “When he transplanted someone, that person was his patient for life.”

On its LinkedIn page, the University of Texas Health Science Center once featured a photo of a billboard with Dr. Bynon on it. The sign read, “Dr. Bynon gives new life to transplant patients.”

Dr. Bynon also served on the Membership and Professional Standards Committee of the United Network for Organ Sharing, which investigates wrongdoing in the transplant system.

Most recently, in December, Dr. Bynon made headlines for performing a kidney transplant for former Lt. Gov. Ben Barnes of Texas.

The closure of the programs at Memorial Hermann has surprised many in the transplant community because it is extremely rare for a program to be suspended over ethical issues.

At the time it shut down its programs, Memorial Hermann had 38 patients on its liver transplant waiting list and 346 patients on its kidney list, according to the hospital.

Officials said they were contacting those patients to help them find new providers.

Roni Caryn Rabin contributed reporting. Susan C. Beachy and Kirsten Noyes contributed research.