Lawyers for the passenger dragged violently from a United plane appear to be launching legal action against the airline.

Footage of a bloodied Dr David Dao, 69, being dragged off an overbooked flight by airport security officers on Sunday night quickly went viral.

On Wednesday, his lawyers filed an emergency request with an Illinois court to make sure that United preserves evidence such as videos, cockpit voice recordings, passenger and crew lists and other materials related to United Flight 3411.

The papers, filed in Cook County courthouse, said: "After being duly processed by the ticket agent, checked in by the attendant and seated in his assigned passenger seat, Petitioner was forcibly dragged and removed from the said aircraft by City employees, sustaining personal injury."

A spokesman for Corboy & Demetrio, one of the firms acting on behalf of the doctor, said he is still being treated in hospital but one of his relatives is expected to give a news conference on Thursday in Chicago.


Man removed from United Airlines Flight

Videos of Mr Dao's treatment caused millions of dollars to be wiped off the value of the United's parent company United Continental Holdings and social media users from around the world called for a boycott of the airline.

Much of the outrage centred on the fact that Mr Dao was a paying passenger who had been removed from the Chicago to Louisville flight to make way for additional crew members.

United's chief executive Oscar Munoz apologised on Wednesday and said that police would not be used to remove passengers from overbooked flights in future.

He said he had not spoken to Mr Dao but added: "I do look forward to a time when I can, as much as I'm able to, apologise directly to him for what has happened."

Can United Airlines beat the backlash?

Mr Munoz was criticised for his initial response to the incident, where he said the passenger had "defied" authorities and "compounded" the situation.

PR Week, which named Mr Munoz its US communicator of the year just last month, described his response as "tone deaf" and framing the event "purely in terms of its effect on United, rather than the injured passenger".

Mr Munoz also promised every passenger on board the flight would get the price of their ticket refunded.

Three airport security officers have now been put on leave following the incident.