John Bacon, and Ben Mutzabaugh

USA TODAY

The CEO of United Airlines apologized again Tuesday amid a global uproar sparked when a passenger was dragged screaming from his seat on a flight that, it turns out, wasn't even overbooked.

"I continue to be disturbed by what happened on this flight, and I deeply apologize to the customer forcibly removed and to all the customers aboard," CEO Oscar Munoz said in a statement. "No one should ever be mistreated this way."

United has been under siege since videos of Sunday night's violent confrontation on the plane at Chicago's O'Hare Airport went viral, drawing hundreds of millions of views around the world. Social media outrage rained down on the Chicago-based airline.

United spokesman Jonathan Guerin said Tuesday that all 70 seats on United Express Flight 3411 were filled, but the plane was not overbooked as the airline previously reported. Instead, United and regional affiliate Republic Airlines, which operated the flight, selected four passengers to be removed to accommodate crew members needed in Louisville the next day. The passengers were selected based on a combination of criteria spelled out in United’s contract of carriage, including frequent-flier status, fare type, check-in time and connecting flight implications, among others, according to United.

Three passengers went quietly. The fourth, who was literally pulled out of his seat and off the plane, was David Dao, a physician in Elizabethtown, Ky.

Late Tuesday, CNBC reported that a pair of Chicago attorneys, Stephen L. Golan and Thomas A. Demetrio, are representing Dao. A statement from Golan said Dao is undergoing treatment in a Chicago hospital for unspecified injuries.

"The family of Dr. Dao wants the world to know that they are very appreciative of the outpouring of prayers, concern, and support they have received," Golan said in a statement. "Currently, they are focused only on Dr. Dao's medical care and treatment" and will not be making any comments to the media.

Munoz issued his first public apology Monday but hours later sent a letter to the airline's employees lauding the behavior of the flight crew in dealing with a "disruptive and belligerent" passenger. Munoz credited employees with following established procedures on the Louisville-bound flight.

"This situation was unfortunately compounded when one of the passengers we politely asked to deplane refused, and it became necessary to contact Chicago Aviation Security Officers to help," the letter says. "While I deeply regret this situation arose, I also emphatically stand behind all of you, and I want to commend you for continuing to go above and beyond to ensure we fly right."

Munoz conceded, however, that "there are lessons we can learn from this experience," and he promised an investigation. Chicago aviation officials placed a security officer on leave, saying the incident "was not in accordance with our standard operating procedure."

UNITED TAKES A HIT

Eric Schiffer, CEO of Reputation Management Consultants, termed United's handling of the incident "brand suicide."

"When you go onto a United flight, you shouldn’t have to be concerned there will be blood or you will get slammed in the face," Schiffer said. "I think you will see an effect on sales from those who are disgusted by the gruesome action. And it’s catastrophic for a brand’s trust."

United stock had dropped steadily by midafternoon Tuesday, amounting to an estimated $255 million of the airline's market value. Schiffer said United, aided by robust advertising, will recover.

"But they are definitely going to lose customers and they should," he said.

The incident came two weeks after United took withering criticism for requiring two girls to change out of leggings before boarding a flight in Denver. United explained that leggings and yoga pants are among banned attire for people flying with employee pass privileges because such fliers are viewed as representing the company.

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CONFRONTATION TURNS VIOLENT

United spokeswoman Erin Benson said four crew members needed seats on Sunday's flight or another United flight was in danger of being canceled. United offered passengers $1,000 in compensation if they agreed to take a different plane to Louisville but drew no takers.

Passenger Jason Powell said Dao was calm when asked to exit the plane.

"He was very polite, matter-of-fact," Powell said. "I could hear pretty clearly. He was acting appropriately annoyed. I was 100% with him — I wouldn't have gotten off the plane either."

Passenger Audra Bridges said when United officials pressed the issue, Dao became "very upset," saying he was a doctor who needed to meet with patients the next morning. Security officers came and Dao was forcibly removed from the plane.

Bridges posted a video of the incident online Sunday evening, which shows three security officers speaking to an unidentified passenger. One of the men grabs the passenger, who screams as he is yanked out of his seat and pulled down the aisle.

The man managed to get back on the flight after that, Bridges said. She said his face was bloody and he seemed disoriented. Another video that was shared on social media appeared to show the same man who was dragged off the flight rushing to the back of the plane and repeatedly saying, "I have to go home."

The videos went viral and the story roared into national headlines. The videos drew more than 200 million views in China alone, with many Chinese viewers expressing outrage online about possible ethnic bias.

DETAILS EMERGE ON PASSENGER

More details on the passenger kicked off the plane emerged Tuesday. Dao, who came to the U.S after attending medical school in Vietnam in the 1970s, was convicted in 2004 on drug-related offenses after an undercover investigation, according to documents filed with the state Board of Medical Licensure. He surrendered his medical license in 2005, but the Kentucky board permitted Dao to resume practicing medicine in 2015 on a limited basis.

Schiffer said a criminal case dating back more than a decade is no excuse for Dao being "treated like a beaten animal."

"The poor guy, I really do feel for him," Powell said. "This didn't need to happen. I'm sure he didn't expect it to happen to him. I wouldn't have expected that to happen to me."

Dao has played professional poker and has a World Series of Poker profile, according to the organization's web site. His profile shows that he has a total earnings of $234,664 at the card table. Dao won $117,744 at a Tunica, Miss. WSOP circuit championship in 2009, WSOP records show.

Contributing: Morgan Watkins, Lucas Aulbach andBailey Loosemore, The (Louisville) Courier Journal; Associated Press