The CEO of United Airlines has said that employees “followed established procedures” in an incident which saw a passenger dragged off a plane.
In a letter to employees obtained by US media, Oscar Munoz said he was “upset to see and hear about what happened”.
But he said that the passenger had been “disruptive and belligerent”.
The airline earlier said that it was investigating what happened after videos of the incident provoked an outcry on social media.
The footage taken inside the airliner shows a man being violently pulled out of his seat and dragged down the aisle.
The flight from Chicago to Louisville on Sunday evening had been overbooked.
The airline wanted to get four passengers to leave the flight to make room for four staff members.
Some Twitter users criticised Mr Munoz for appearing tone-deaf to the level of public outcry.
He told staff in the private email that he was “upset to see and hear about what happened”.
“Our employees followed established procedures for dealing with situations like this,” the Associated Press quoted the email as saying.
“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.”
Mr Munoz wrote that the passenger refused to voluntarily leave the plane, with staff “left with no choice but to call Chicago Aviation Security Officers to assist in removing the customer from the flight”.
Jayse D Anspach, who posted the footage that went viral, tweeted: “#United overbooked and wanted four of us to volunteer to give up our seats for personnel that needed to be at work the next day.”
“No one volunteered, so United decided to choose for us. They chose an Asian doctor and his wife.”
“The doctor needed to work at the hospital the next day, so he refused to volunteer,” Mr Anspach added.
“Ten minutes later, the doctor runs back into the plane with a bloody face, clings to a post in the back, chanting, “I need to go home.”
One of the three security officers involved has been “placed on leave”, the Chicago Department of Aviation said, and his actions were “obviously not condoned by the Department”.
The department also said it would carry out a review into the incident, which it said was “not in accordance with our standard operating procedure”.
Another passenger Audra D. Bridges, posted a video of the incident on Facebook that has been viewed over 400,000 times.
She wrote: “Please share this video. We are on this flight. United airlines overbooked the flight.”
“They randomly selected people to kick off so their standby crew could have a seat.
“This man is a doctor and has to be at the hospital in the morning,” she added.
“He did not want to get off. We are all shaky and so disgusted.”
United said it was trying to talk to the passenger directly in order to “further address and resolve this situation”.