CHICAGO >> After people were horrified by video of a passenger getting dragged off a full United Express flight by airport police, the head of United’s parent company said the airline was reaching out to the man to “resolve this situation.”

Hours later on Monday, his tone turned defensive. He described the man as “disruptive and belligerent.”

By this afternoon, almost two days after the Sunday evening confrontation in Chicago, CEO Oscar Munoz issued his most contrite apology yet as details emerged about the man seen on cellphone videos recorded by other passengers at O’Hare Airport.

“No one should ever be mistreated this way,” Munoz said.

The passenger was identified as physician David Dao, 69, of Elizabethtown, Kentucky, who was convicted more than a decade ago of felony charges involving his prescribing of drugs and spent years trying to regain his medical license.

But while Dao’s history quickly became a focus of attention, there’s no indication that his past influenced how he was treated or that the airline or police were aware of his background or would have known anything about him other than basic information such as his name and address, if that.

Screaming can be heard on the videos, but nowhere is Dao seen attacking the officers. In fact, he appears relatively passive both when he was dragged down the aisle of the jet and when he is seen standing in the aisle later saying quietly, “I want to go home, I want to go home.”

Munoz’s latest statement described the removal as “truly horrific.” He said the company would review policies for seeking volunteers to give up their seats, for handling oversold situations and for partnering with airport authorities and local law enforcement.

An attorney who represents Dao said his client was being treated at a Chicago hospital for injuries he sustained on the plane and that the family would not comment.

According to records from the Kentucky Board of Medical Licensure, Dao went to medical school at the University of Medicine of Ho Chi Minh City in Vietnam, graduating in 1974. He was licensed in Kentucky with a specialty in pulmonary disease.

His legal troubles started in 2003, when his medical license was suspended after an undercover sting operation at a Louisville motel for allegedly writing fraudulent prescriptions.

According to the documents, the licensing board had learned that Dao had become sexually interested in a patient and hired the patient as his office manager. That man later said he quit his job because Dao “pursued him aggressively” and arranged to provide him with prescription drugs in exchange for sex.

Dao was ultimately convicted in late 2004 of several counts of obtaining drugs by fraud or deceit and was placed on five years of supervised probation and surrendered his medical license.

His longtime effort to get his license back finally succeeded in 2015, when the licensing board allowed him to practice medicine again.

About a year after his medical license was suspended, Dao joined the professional poker circuit, according to his World Series of Poker profile. His biggest competitive win came in 2009 when he took home more than $117,000 from a tournament in Mississippi.

Airport officials have said little about Sunday’s events and nothing about Dao’s behavior before he was pulled from the jet that was bound for Louisville, Kentucky. Likewise, the Chicago Aviation Department has said only that one of its employees who removed Dao did not follow proper procedures and has been placed on leave.

No passengers on the plane have mentioned that Dao did anything but refuse to leave the plane when he was ordered to do so.

Also today, Chicago Mayor Rahm Emanuel called the way Dao was treated “completely unacceptable” and praised Aviation Commissioner Ginger Evans for taking “swift action.” He promised that a city investigation would “ensure nothing like this ever happens again.”

The event stemmed from a common air travel issue — a full flight. United was trying to make room for four employees of a partner airline, meaning four people had to get off.

At first, the airline asked for volunteers, offering $400 and then when that did not work, $800 per passenger to relinquish a seat. When no one voluntarily came forward, United selected four passengers at random.

Three people got off the flight, but the fourth said he was a doctor and needed to get home to treat patients on Monday. He refused to leave.

Three Aviation Department police officers got on the plane. Two officers tried to reason with the man before a third came aboard and pointed at the man “basically saying, ‘Sir, you have to get off the plane,’” said Tyler Bridges, a passenger whose wife, Audra D. Bridges, posted a video on Facebook.

One of the officers could be seen grabbing the screaming man from his window seat, across the armrest and dragging him down the aisle by his arms.

Other passengers on Flight 3411 are heard saying, “Please, my God,” ”What are you doing?” ”This is wrong,” ”Look at what you did to him” and “Busted his lip.”

The U.S. Department of Transportation announced today that it is reviewing Sunday’s events to see if United violated rules on overselling flights.

Dao’s relatives are focused only on his medical care, attorney Stephen L. Golan said. The family “wants the world to know that they are very appreciative of the outpouring of prayers, concern and support they have received.”