NEW YORK (REUTERS) - United Airlines sparked outrage on Monday (April 10) for the treatment of a passenger who was dragged off a plane the airline had overbooked, and one of the security officers involved in the incident was placed on leave pending an investigation.

Videos posted online by other passengers showed a man screaming as officers yanked him from his seat on United Flight 3411 before it departed from Chicago O'Hare International Airport to Louisville, Kentucky, on Sunday.

The man, who appeared to be Asian, was seen being dragged down the aisle on his back by his hands, body limp, bleeding from the mouth, glasses askew and shirt pulled up above his navel.

The videos sparked outrage on social media, the second time in less than a month that United Airlines was criticised for its treatment of passengers.

The flight was overbooked and the airline asked for volunteers to leave because additional flight crew needed to get to Louisville. No one volunteered and the airline picked four passengers randomly. Airline spokesman Charlie Hobart said three of them agreed to get off the flight, but the fourth person refused and police were called.

In a letter circulated to employees and seen by Reuters, United chief executive officer Oscar Munoz did not apologise for the way the passenger was handled, writing that the passenger had "defied" security officers.

Munoz said there are lessons the company can learn from this situation, though he impressed that he "emphatically" stands behind his employees. "We sought volunteers and then followed our involuntary denial of boarding process (including offering up to US$1,000 (S$1,407) in compensation)," Munoz wrote.

"When we approached one of these passengers to explain apologetically that he was being denied boarding, he raised his voice and refused to comply with crew member instructions."

The Chicago Department of Aviation said in a statement that one of the officers did not follow protocol and added that he had been placed on leave pending a review for actions not condoned by the department.

The US Department of Transportation (DOT) said it was reviewing whether United complied with overbook rules that require airlines to set guidelines on how passengers are denied boarding if they do not volunteer to give up their seats.

"While it is legal for airlines to involuntarily bump passengers from an oversold flight when there are not enough volunteers, it is the airline's responsibility to determine its own fair boarding priorities," a DOT spokesman said in a statement.

'JUST KILL ME'

The airline was one of the top-trending topics on Twitter as users took to the website to express their anger over the forceful removal of the passenger. A video of the incident was posted to Twitter account #Tyler-Bridges.

Bridges said the man told United staff that he was a doctor and had to return home to his patients.

The airline's chief executive officer said: "This is an upsetting event to all of us here at United. I apologise for having to re-accommodate these customers."

Many social media users criticised the apology for not directly addressing the forcible removal of the passenger. "Apologise for saying you 'had to' do this. There were other options and you know it," user @TessaDare wrote in a series of posts retweeted thousands of times. "Apologise for creating and allowing a corporate culture that says it's okay to treat passengers with such disregard and disdain."

In Bridges' video, a woman asks: "Can't they rent a car for the pilots and have them drive?" Two uniformed men then reach into the man's seat and snatch him from his chair.

Fellow passenger Jayse D. Anspach, who goes by @JayseDavid on Twitter, wrote: "No one volunteered (to leave), so @United decided to choose for us. They chose an Asian doctor and his wife."

"It looked like he was knocked out, because he went limp and quiet," Anspach wrote, "and they dragged him out of the plane like a rag doll."

Another video shows the distressed man, still dishevelled from the wrangle, returned to the cabin, clinging onto a curtain at the back of the plane and repeating: "Just kill me. Kill me," and "I have to go home," as blood streaked down his mouth.

Reports said the man was later taken away on a stretcher and the plane took off after a three-hour delay.

Much of the online uproar surrounded the appropriateness of removing a paying customer in order to accommodate airline staff. "They bloodied a senior citizen & dragged him off the plane so THEIR OWN STAFF could take his seat," one Twitter user wrote.

Other social media users questioned whether the man would have been removed as forcefully had he not been Asian.

Late last month, two teenage girls dressed in leggings were denied boarding on a United flight from Denver to