Man dragged off United flight plans to sue: Lawyers

US civil rights activist Reverend Jesse Jackson led a protest against United Airlines at O'Hare airport over the violent dragging of a paid passenger from his seat to accommodate the airline's overbooking of the flight. (AFP/Joshua LOTT)

David Dao was released from the hospital overnight and was at a "secure location," attorney Thomas Demetrio said at a news conference during which a member of Dao's family spoke out for the first time.

United remained under a spotlight on Thursday as representatives of the carrier faced tough questioning at a city council hearing in Chicago, where the airline is headquartered and where the incident occurred.

Dao's lawyers filed a petition with a Chicago court requesting that the city, which operates O'Hare International Airport, and United Airlines preserve evidence related to the incident on Sunday. They also said a lawsuit was forthcoming.

"This lawsuit, among other things, hopefully, will create a not just national discussion, but international discussion, on how we're going to be treated going forward," Demetrio said. "For a long time, airlines, United in particular, have bullied us."

Online video of airport security officers on Sunday dragging Dao off a packed flight sparked worldwide outrage. He could be seen screaming as officers pulled him from his seat, and was bloodied by the altercation.

The 69-year-old doctor's lawyers said he also suffered injury to his sinuses and lost two front teeth.

"My dad is healing right now," said Crystal Dao Pepper, 33, one of Dao's five children. "We were completely horrified and shocked at what had happened to my father," she said.

In response, United Airlines released a statement reiterating its apology.

"We continue to express our sincerest apology to Dr Dao. We cannot stress enough that we remain steadfast in our commitment to make this right," the statement said.

The airline added that it would no longer ask law enforcement to remove a passenger from a flight unless it is a matter of safety and security.

The statement did not quell criticism of United, especially since its apologies came days after the incident - and after initial statements appeared to at least in part blame Dao.

At a sometimes tense hearing on Thursday at Chicago's city hall, officials from United Airlines and O'Hare airport said they are conducting investigations to determine what went wrong with their procedures.

United said it will release the results of its review on Apr 30.

"We commit to you that this type of situation will never happen again aboard our aircraft," Margaret Smith, United's head of corporate affairs, said at the hearing.

Asked why United took days to apologise, Smith said the airline made a mistake.

"We took too long to say anything, and our statement - when it first came out - did not show the depth of our concern and regret," Smith said.

"We should have handled it quicker, and we should have been better at expressing how that is something that we just do not want ever to happen again at United."

Lawmakers in Congress have also signalled potential action.

Congresswoman Jan Schakowsky of Illinois said she would author a bill to end the practice of airlines denying boarding to passengers on overbooked flights. A group of 21 senators also said they planned to examine the incident.