The United Airlines flight that drew worldwide attention after a passenger was dragged from his seat and off the plane was not overbooked, the airline said Tuesday.

United spokesman Jonathan Guerin said the flight was sold out — but not oversold. Instead, United and regional affiliate Republic Airlines – the unit that operated Flight 3411 – decided they had to remove four passengers from the flight to accommodate crewmembers who were needed in Louisville the next day for a “downline connection.”

"They were considered 'must-ride' passengers," Guerin told USA TODAY.

United has been under siege since videos of Sunday night's violent struggle on the plane at Chicago's O'Hare Airport went viral. CEO Oscar Munoz late Monday sent a letter to the airline's employees lauding the behavior of the flight crew when a "disruptive and belligerent" Kentucky physician was dragged off a plane in Chicago.

Videos of Sunday's violent confrontation and the bloodied passenger went viral, drawing hundreds of millions of views around the world. Social media outrage rained down on the Chicago-based airline, prompting a public apology Monday from CEO Oscar Munoz. But in the letter sent to employees hours later and obtained by numerous media outlets, Munoz credited employees with following established procedures on Louisville-bound United Express Flight 3411.

Four passengers were selected at random to give up their seats. Three did so. The man literally pulled out of his seat and off the plane was David Dao, a physician in Elizabethtown, Ky.

"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."

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.

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 up to $800 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 that, 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.

By Monday afternoon, Munoz had issued a public, apologetic statement about this "upsetting event."

On Tuesday, Dao's background emerged on the national news cycle. 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 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."

Watkins and Aulbach write for the The (Louisville) Courier-Journal; Bacon for USA TODAY. Contributing: Bailey Loosemore, The Courier Journal;  Associated Press