David Dao, 69, who was dragged from a United Airlines flight on Sunday, is a Kentucky-based doctor

Revealed: All About the Doctor Dragged Off Overbooked United Flight — and His Troubled Past

United Airlines is in the midst of a media storm after officials violently dragged a man from an overbooked flight on Sunday after telling him to give up his seat.

David Dao, 69, was one of four passengers asked to leave an oversold Louisville-bound flight from Chicago’s O’Hare International Airport so airline employees could board the plane. The altercation saw Dao bloodied and bruised as officials with the Chicago Department of Aviation dragged him through the aisle of the aircraft.

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“When we didn’t get the number of volunteers we needed then we had to follow Department of Transportation procedures and ask that customers, four of them, exit the aircraft,” United spokesman Charlie Hobart told PEOPLE.

“One customer refused to do that after we had explained the situation and apologized and let that customer know what we were gonna do on his behalf to get that customer to where he needed to be. He still refused, at which point we had to involve the Chicago Police Department.”

Passengers were offered $400 and a hotel stay to give up their seats to stand-by United employees heading to Louisville for a flight, fellow traveler Audra D. Bridges told the Courier-Journal. When no one volunteered, the offer was increased to $800.

Dao’s daughter, Crystal Pepper, along with attorneys Thomas A. Demetrio and Stephen L. Golan, spoke out at a press conference Thursday in Chicago saying they are working on a lawsuit, but have not filed anything yet.

“What happened to my dad should never have happened to any human being, regardless of the circumstances,” Pepper told reporters. “We were horrified and shocked and sickened to see what had happened to him.”

Here are four more things to know about the headline-making incident:

1. Facebook footage of the incident drew the ire of social media users everywhere — with his lawyers calling it a “harrowing” experience

Audra Bridges uploaded a video of the incident to Facebook, writing that she and the other passengers were “disgusted.”

Footage showed Dao being thrown against an armrest by security and dragged as one woman screamed, “Oh my God! Look at what you did to him.”

Anther clip taken by fellow passenger Joya Griffin Cummings, shows Dao speaking to security personnel who are telling him to deplane after the airline overbooked the flight.

“I won’t go, I’m a physician I have to work tomorrow, 8 o’clock…” Dao says.

“I was like, ‘Uh oh, this is not great,’ ” Griffin Cummings, 37, tells PEOPLE. “It made me more anxious. The space is so small and we didn’t know if they had weapons or what.”

Many Twitter users took to the social media site to condemn the airline, with one person calling the situation “horrific.”

And on Thursday Dao’s attorney Thomas A. Demetrio said his client, who immigrated from Vietnam in 1975 after “Saigon fell,” was more “terrified” during his altercation on United than he was of leaving Saigon.

“He said that being dragged down the aisle was more horrifying and harrowing than what he experienced in leaving Vietnam,” says Demetrio.

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2. Dao is a Kentucky doctor — and his wife is a pediatrician.

In the wake of the incident, details about Dao’s life have come to light, including the fact that he is a doctor in Kentucky.

He reportedly told officials that he is a doctor and could not leave the flight because he had patients to see in Louisville early Monday morning, according to the Courier-Journal.

His license permits him to practice internal medicine at an outpatient facility once a week, according to documents obtained by PEOPLE.

Dao’s wife, Teresa, works as a pediatrician in Elizabethtown.

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3. Dao has a troubled past.

Dao was arrested in 2003 as part of an undercover operation. Two years later, Dao was convicted on six felony counts of obtaining drugs by fraud and deceit and in 2005, he was sentenced to five years probation. Dao was also convicted for writing prescriptions and checks to a male patient in exchange for sexual favors.

In February 2005, Dao surrendered his license to practice medicine in Kentucky.

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In response, the state medical licensing board issued a suspension that was lifted in 2015. But the board has since placed severe restrictions on Dao’s ability to practice internal medicine, which will be lifted on Feb. 28, 2018, according to documents obtained by PEOPLE.

State records indicate the board believes Dao’s practices are outdated.

Last year, the medical board imposed restrictions on his right to practice. He can only practice internal medicine in an outpatient facility one day a week.

4. United Airlines CEO sparked uproar after he issued a statement apologizing “for having to re-accommodate these customers.”

In a statement, United CEO Oscar Munoz called the incident “upsetting.”

“I apologize for having to re-accommodate these customers. Our team is moving with a sense of urgency to work with the authorities and conduct our own detailed review of what happened,” Munoz said in the statement, according to NBC Chicago.

Munoz added: “We are also reaching out to this passenger to talk directly to him and further address and resolve this situation.”

But in an email to employees on Monday, the United Airlines CEO faulted Dao for being “disruptive and belligerent.”

“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,” CEO Oscar Munoz said in email to employees, reported by CNBC. “Our employees followed established procedures for dealing with situations like this. While I deeply regret this situation arose, I also emphatically stand behind all of you.”

In recounting the sequence of events, Munoz told employees that the passenger “refused” to deplane and “became more and more disruptive and belligerent” and faulted him for “running back onto the aircraft in defiance of both our crew and security officials.”

Munoz doubled down on his apology on Wednesday, telling ABC News: “That is not who our family at United is. You saw us at a bad moment. This can never, will never happen again on a United Airlines flight … that’s my promise.”

Munoz said he is “ashamed” as a result of the incident, calling the situation a “system failure.”

“As I think about our business and our people the first thing I think is important to say is to apologize to Dr. Dao, his family, the passengers on that flight, our customers, our employees,” he said.

“We are not going to put a law enforcement official onto a plane to take them off … to remove a booked, paid, seated passenger; we can’t do that.”